{-# LANGUAGE CPP #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE MultiWayIf #-}

module GHC.Tc.Solver.Canonical(
     canonicalize,
     unifyDerived, unifyTest, UnifyTestResult(..),
     makeSuperClasses,
     StopOrContinue(..), stopWith, continueWith, andWhenContinue,
     solveCallStack    -- For GHC.Tc.Solver
  ) where

#include "GhclibHsVersions.h"

import GHC.Prelude

import GHC.Tc.Types.Constraint
import GHC.Core.Predicate
import GHC.Tc.Types.Origin
import GHC.Tc.Utils.Unify
import GHC.Tc.Utils.TcType
import GHC.Core.Type
import GHC.Tc.Solver.Rewrite
import GHC.Tc.Solver.Monad
import GHC.Tc.Types.Evidence
import GHC.Tc.Types.EvTerm
import GHC.Core.Class
import GHC.Core.TyCon
import GHC.Core.Multiplicity
import GHC.Core.TyCo.Rep   -- cleverly decomposes types, good for completeness checking
import GHC.Core.Coercion
import GHC.Core.Coercion.Axiom
import GHC.Core
import GHC.Types.Id( mkTemplateLocals )
import GHC.Core.FamInstEnv ( FamInstEnvs )
import GHC.Tc.Instance.Family ( tcTopNormaliseNewTypeTF_maybe )
import GHC.Types.Var
import GHC.Types.Var.Env( mkInScopeSet )
import GHC.Types.Var.Set( delVarSetList, anyVarSet )
import GHC.Utils.Outputable
import GHC.Utils.Panic
import GHC.Builtin.Types ( anyTypeOfKind )
import GHC.Driver.Session( DynFlags )
import GHC.Types.Name.Set
import GHC.Types.Name.Reader
import GHC.Hs.Type( HsIPName(..) )

import GHC.Data.Pair
import GHC.Utils.Misc
import GHC.Data.Bag
import GHC.Utils.Monad
import Control.Monad
import Data.Maybe ( isJust, isNothing )
import Data.List  ( zip4, partition )
import GHC.Types.Unique.Set( nonDetEltsUniqSet )
import GHC.Types.Basic

import Data.Bifunctor ( bimap )
import Data.Foldable ( traverse_ )

{-
************************************************************************
*                                                                      *
*                      The Canonicaliser                               *
*                                                                      *
************************************************************************

Note [Canonicalization]
~~~~~~~~~~~~~~~~~~~~~~~

Canonicalization converts a simple constraint to a canonical form. It is
unary (i.e. treats individual constraints one at a time).

Constraints originating from user-written code come into being as
CNonCanonicals. We know nothing about these constraints. So, first:

     Classify CNonCanoncal constraints, depending on whether they
     are equalities, class predicates, or other.

Then proceed depending on the shape of the constraint. Generally speaking,
each constraint gets rewritten and then decomposed into one of several forms
(see type Ct in GHC.Tc.Types).

When an already-canonicalized constraint gets kicked out of the inert set,
it must be recanonicalized. But we know a bit about its shape from the
last time through, so we can skip the classification step.

-}

-- Top-level canonicalization
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

canonicalize :: Ct -> TcS (StopOrContinue Ct)
canonicalize :: Ct -> TcS (StopOrContinue Ct)
canonicalize (CNonCanonical { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev })
  = {-# SCC "canNC" #-}
    CtEvidence -> TcS (StopOrContinue Ct)
canNC CtEvidence
ev

canonicalize (CQuantCan (QCI { qci_ev :: QCInst -> CtEvidence
qci_ev = CtEvidence
ev, qci_pend_sc :: QCInst -> Bool
qci_pend_sc = Bool
pend_sc }))
  = CtEvidence -> Bool -> TcS (StopOrContinue Ct)
canForAll CtEvidence
ev Bool
pend_sc

canonicalize (CIrredCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev })
  = CtEvidence -> TcS (StopOrContinue Ct)
canNC CtEvidence
ev
    -- Instead of rewriting the evidence before classifying, it's possible we
    -- can make progress without the rewrite. Try this first.
    -- For insolubles (all of which are equalities), do /not/ rewrite the arguments
    -- In #14350 doing so led entire-unnecessary and ridiculously large
    -- type function expansion.  Instead, canEqNC just applies
    -- the substitution to the predicate, and may do decomposition;
    --    e.g. a ~ [a], where [G] a ~ [Int], can decompose

canonicalize (CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev, cc_class :: Ct -> Class
cc_class  = Class
cls
                       , cc_tyargs :: Ct -> [Xi]
cc_tyargs = [Xi]
xis, cc_pend_sc :: Ct -> Bool
cc_pend_sc = Bool
pend_sc })
  = {-# SCC "canClass" #-}
    CtEvidence -> Class -> [Xi] -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
ev Class
cls [Xi]
xis Bool
pend_sc

canonicalize (CEqCan { cc_ev :: Ct -> CtEvidence
cc_ev     = CtEvidence
ev
                     , cc_lhs :: Ct -> CanEqLHS
cc_lhs    = CanEqLHS
lhs
                     , cc_rhs :: Ct -> Xi
cc_rhs    = Xi
rhs
                     , cc_eq_rel :: Ct -> EqRel
cc_eq_rel = EqRel
eq_rel })
  = {-# SCC "canEqLeafTyVarEq" #-}
    CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqNC CtEvidence
ev EqRel
eq_rel (CanEqLHS -> Xi
canEqLHSType CanEqLHS
lhs) Xi
rhs

canNC :: CtEvidence -> TcS (StopOrContinue Ct)
canNC :: CtEvidence -> TcS (StopOrContinue Ct)
canNC CtEvidence
ev =
  case Xi -> Pred
classifyPredType Xi
pred of
      ClassPred Class
cls [Xi]
tys     -> do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:cls" (Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
<+> [Xi] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Xi]
tys)
                                  CtEvidence -> Class -> [Xi] -> TcS (StopOrContinue Ct)
canClassNC CtEvidence
ev Class
cls [Xi]
tys
      EqPred EqRel
eq_rel Xi
ty1 Xi
ty2 -> do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:eq" (Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty1 SDoc -> SDoc -> SDoc
$$ Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty2)
                                  CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqNC    CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ty2
      IrredPred {}          -> do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:irred" (Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
pred)
                                  CtEvidence -> TcS (StopOrContinue Ct)
canIrred CtEvidence
ev
      ForAllPred [TyVar]
tvs [Xi]
th Xi
p   -> do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:forall" (Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
pred)
                                  CtEvidence -> [TyVar] -> [Xi] -> Xi -> TcS (StopOrContinue Ct)
canForAllNC CtEvidence
ev [TyVar]
tvs [Xi]
th Xi
p
  where
    pred :: Xi
pred = CtEvidence -> Xi
ctEvPred CtEvidence
ev

{-
************************************************************************
*                                                                      *
*                      Class Canonicalization
*                                                                      *
************************************************************************
-}

canClassNC :: CtEvidence -> Class -> [Type] -> TcS (StopOrContinue Ct)
-- "NC" means "non-canonical"; that is, we have got here
-- from a NonCanonical constraint, not from a CDictCan
-- Precondition: EvVar is class evidence
canClassNC :: CtEvidence -> Class -> [Xi] -> TcS (StopOrContinue Ct)
canClassNC CtEvidence
ev Class
cls [Xi]
tys
  | CtEvidence -> Bool
isGiven CtEvidence
ev  -- See Note [Eagerly expand given superclasses]
  = do { [Ct]
sc_cts <- CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [] [] Class
cls [Xi]
tys
       ; [Ct] -> TcS ()
emitWork [Ct]
sc_cts
       ; CtEvidence -> Class -> [Xi] -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
ev Class
cls [Xi]
tys Bool
False }

  | CtEvidence -> Bool
isWanted CtEvidence
ev
  , Just FastString
ip_name <- Class -> [Xi] -> Maybe FastString
isCallStackPred Class
cls [Xi]
tys
  , OccurrenceOf Name
func <- CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc
  -- If we're given a CallStack constraint that arose from a function
  -- call, we need to push the current call-site onto the stack instead
  -- of solving it directly from a given.
  -- See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence
  -- and Note [Solving CallStack constraints] in GHC.Tc.Solver.Monad
  = do { -- First we emit a new constraint that will capture the
         -- given CallStack.
       ; let new_loc :: CtLoc
new_loc = CtLoc -> CtOrigin -> CtLoc
setCtLocOrigin CtLoc
loc (HsIPName -> CtOrigin
IPOccOrigin (FastString -> HsIPName
HsIPName FastString
ip_name))
                            -- We change the origin to IPOccOrigin so
                            -- this rule does not fire again.
                            -- See Note [Overview of implicit CallStacks]

       ; CtEvidence
new_ev <- CtLoc -> Xi -> TcS CtEvidence
newWantedEvVarNC CtLoc
new_loc Xi
pred

         -- Then we solve the wanted by pushing the call-site
         -- onto the newly emitted CallStack
       ; let ev_cs :: EvCallStack
ev_cs = Name -> RealSrcSpan -> EvExpr -> EvCallStack
EvCsPushCall Name
func (CtLoc -> RealSrcSpan
ctLocSpan CtLoc
loc) (CtEvidence -> EvExpr
ctEvExpr CtEvidence
new_ev)
       ; CtEvidence -> EvCallStack -> TcS ()
solveCallStack CtEvidence
ev EvCallStack
ev_cs

       ; CtEvidence -> Class -> [Xi] -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
new_ev Class
cls [Xi]
tys Bool
False }

  | Bool
otherwise
  = CtEvidence -> Class -> [Xi] -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
ev Class
cls [Xi]
tys (Class -> Bool
has_scs Class
cls)

  where
    has_scs :: Class -> Bool
has_scs Class
cls = Bool -> Bool
not ([Xi] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null (Class -> [Xi]
classSCTheta Class
cls))
    loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    pred :: Xi
pred = CtEvidence -> Xi
ctEvPred CtEvidence
ev

solveCallStack :: CtEvidence -> EvCallStack -> TcS ()
-- Also called from GHC.Tc.Solver when defaulting call stacks
solveCallStack :: CtEvidence -> EvCallStack -> TcS ()
solveCallStack CtEvidence
ev EvCallStack
ev_cs = do
  -- We're given ev_cs :: CallStack, but the evidence term should be a
  -- dictionary, so we have to coerce ev_cs to a dictionary for
  -- `IP ip CallStack`. See Note [Overview of implicit CallStacks]
  EvExpr
cs_tm <- EvCallStack -> TcS EvExpr
forall (m :: * -> *).
(MonadThings m, HasModule m, HasDynFlags m) =>
EvCallStack -> m EvExpr
evCallStack EvCallStack
ev_cs
  let ev_tm :: EvTerm
ev_tm = EvExpr -> TcCoercion -> EvTerm
mkEvCast EvExpr
cs_tm (Xi -> TcCoercion
wrapIP (CtEvidence -> Xi
ctEvPred CtEvidence
ev))
  CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev EvTerm
ev_tm

canClass :: CtEvidence
         -> Class -> [Type]
         -> Bool            -- True <=> un-explored superclasses
         -> TcS (StopOrContinue Ct)
-- Precondition: EvVar is class evidence

canClass :: CtEvidence -> Class -> [Xi] -> Bool -> TcS (StopOrContinue Ct)
canClass CtEvidence
ev Class
cls [Xi]
tys Bool
pend_sc
  =   -- all classes do *nominal* matching
    ASSERT2( ctEvRole ev == Nominal, ppr ev $$ ppr cls $$ ppr tys )
    do { ([Xi]
xis, [TcCoercion]
cos) <- CtEvidence -> TyCon -> [Xi] -> TcS ([Xi], [TcCoercion])
rewriteArgsNom CtEvidence
ev TyCon
cls_tc [Xi]
tys
       ; let co :: TcCoercion
co = Role -> TyCon -> [TcCoercion] -> TcCoercion
mkTcTyConAppCo Role
Nominal TyCon
cls_tc [TcCoercion]
cos
             xi :: Xi
xi = Class -> [Xi] -> Xi
mkClassPred Class
cls [Xi]
xis
             mk_ct :: CtEvidence -> Ct
mk_ct CtEvidence
new_ev = CDictCan :: CtEvidence -> Class -> [Xi] -> Bool -> Ct
CDictCan { cc_ev :: CtEvidence
cc_ev = CtEvidence
new_ev
                                     , cc_tyargs :: [Xi]
cc_tyargs = [Xi]
xis
                                     , cc_class :: Class
cc_class = Class
cls
                                     , cc_pend_sc :: Bool
cc_pend_sc = Bool
pend_sc }
       ; StopOrContinue CtEvidence
mb <- CtEvidence -> Xi -> TcCoercion -> TcS (StopOrContinue CtEvidence)
rewriteEvidence CtEvidence
ev Xi
xi TcCoercion
co
       ; String -> SDoc -> TcS ()
traceTcS String
"canClass" ([SDoc] -> SDoc
vcat [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
                                   , Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
xi, StopOrContinue CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr StopOrContinue CtEvidence
mb ])
       ; StopOrContinue Ct -> TcS (StopOrContinue Ct)
forall (m :: * -> *) a. Monad m => a -> m a
return ((CtEvidence -> Ct)
-> StopOrContinue CtEvidence -> StopOrContinue Ct
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap CtEvidence -> Ct
mk_ct StopOrContinue CtEvidence
mb) }
  where
    cls_tc :: TyCon
cls_tc = Class -> TyCon
classTyCon Class
cls

{- Note [The superclass story]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We need to add superclass constraints for two reasons:

* For givens [G], they give us a route to proof.  E.g.
    f :: Ord a => a -> Bool
    f x = x == x
  We get a Wanted (Eq a), which can only be solved from the superclass
  of the Given (Ord a).

* For wanteds [W], and deriveds [WD], [D], they may give useful
  functional dependencies.  E.g.
     class C a b | a -> b where ...
     class C a b => D a b where ...
  Now a [W] constraint (D Int beta) has (C Int beta) as a superclass
  and that might tell us about beta, via C's fundeps.  We can get this
  by generating a [D] (C Int beta) constraint.  It's derived because
  we don't actually have to cough up any evidence for it; it's only there
  to generate fundep equalities.

See Note [Why adding superclasses can help].

For these reasons we want to generate superclass constraints for both
Givens and Wanteds. But:

* (Minor) they are often not needed, so generating them aggressively
  is a waste of time.

* (Major) if we want recursive superclasses, there would be an infinite
  number of them.  Here is a real-life example (#10318);

     class (Frac (Frac a) ~ Frac a,
            Fractional (Frac a),
            IntegralDomain (Frac a))
         => IntegralDomain a where
      type Frac a :: *

  Notice that IntegralDomain has an associated type Frac, and one
  of IntegralDomain's superclasses is another IntegralDomain constraint.

So here's the plan:

1. Eagerly generate superclasses for given (but not wanted)
   constraints; see Note [Eagerly expand given superclasses].
   This is done using mkStrictSuperClasses in canClassNC, when
   we take a non-canonical Given constraint and cannonicalise it.

   However stop if you encounter the same class twice.  That is,
   mkStrictSuperClasses expands eagerly, but has a conservative
   termination condition: see Note [Expanding superclasses] in GHC.Tc.Utils.TcType.

2. Solve the wanteds as usual, but do no further expansion of
   superclasses for canonical CDictCans in solveSimpleGivens or
   solveSimpleWanteds; Note [Danger of adding superclasses during solving]

   However, /do/ continue to eagerly expand superclasses for new /given/
   /non-canonical/ constraints (canClassNC does this).  As #12175
   showed, a type-family application can expand to a class constraint,
   and we want to see its superclasses for just the same reason as
   Note [Eagerly expand given superclasses].

3. If we have any remaining unsolved wanteds
        (see Note [When superclasses help] in GHC.Tc.Types.Constraint)
   try harder: take both the Givens and Wanteds, and expand
   superclasses again.  See the calls to expandSuperClasses in
   GHC.Tc.Solver.simpl_loop and solveWanteds.

   This may succeed in generating (a finite number of) extra Givens,
   and extra Deriveds. Both may help the proof.

3a An important wrinkle: only expand Givens from the current level.
   Two reasons:
      - We only want to expand it once, and that is best done at
        the level it is bound, rather than repeatedly at the leaves
        of the implication tree
      - We may be inside a type where we can't create term-level
        evidence anyway, so we can't superclass-expand, say,
        (a ~ b) to get (a ~# b).  This happened in #15290.

4. Go round to (2) again.  This loop (2,3,4) is implemented
   in GHC.Tc.Solver.simpl_loop.

The cc_pend_sc flag in a CDictCan records whether the superclasses of
this constraint have been expanded.  Specifically, in Step 3 we only
expand superclasses for constraints with cc_pend_sc set to true (i.e.
isPendingScDict holds).

Why do we do this?  Two reasons:

* To avoid repeated work, by repeatedly expanding the superclasses of
  same constraint,

* To terminate the above loop, at least in the -XNoRecursiveSuperClasses
  case.  If there are recursive superclasses we could, in principle,
  expand forever, always encountering new constraints.

When we take a CNonCanonical or CIrredCan, but end up classifying it
as a CDictCan, we set the cc_pend_sc flag to False.

Note [Superclass loops]
~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
  class C a => D a
  class D a => C a

Then, when we expand superclasses, we'll get back to the self-same
predicate, so we have reached a fixpoint in expansion and there is no
point in fruitlessly expanding further.  This case just falls out from
our strategy.  Consider
  f :: C a => a -> Bool
  f x = x==x
Then canClassNC gets the [G] d1: C a constraint, and eager emits superclasses
G] d2: D a, [G] d3: C a (psc).  (The "psc" means it has its sc_pend flag set.)
When processing d3 we find a match with d1 in the inert set, and we always
keep the inert item (d1) if possible: see Note [Replacement vs keeping] in
GHC.Tc.Solver.Interact.  So d3 dies a quick, happy death.

Note [Eagerly expand given superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In step (1) of Note [The superclass story], why do we eagerly expand
Given superclasses by one layer?  (By "one layer" we mean expand transitively
until you meet the same class again -- the conservative criterion embodied
in expandSuperClasses.  So a "layer" might be a whole stack of superclasses.)
We do this eagerly for Givens mainly because of some very obscure
cases like this:

   instance Bad a => Eq (T a)

   f :: (Ord (T a)) => blah
   f x = ....needs Eq (T a), Ord (T a)....

Here if we can't satisfy (Eq (T a)) from the givens we'll use the
instance declaration; but then we are stuck with (Bad a).  Sigh.
This is really a case of non-confluent proofs, but to stop our users
complaining we expand one layer in advance.

Note [Instance and Given overlap] in GHC.Tc.Solver.Interact.

We also want to do this if we have

   f :: F (T a) => blah

where
   type instance F (T a) = Ord (T a)

So we may need to do a little work on the givens to expose the
class that has the superclasses.  That's why the superclass
expansion for Givens happens in canClassNC.

This same scenario happens with quantified constraints, whose superclasses
are also eagerly expanded. Test case: typecheck/should_compile/T16502b
These are handled in canForAllNC, analogously to canClassNC.

Note [Why adding superclasses can help]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Examples of how adding superclasses can help:

    --- Example 1
        class C a b | a -> b
    Suppose we want to solve
         [G] C a b
         [W] C a beta
    Then adding [D] beta~b will let us solve it.

    -- Example 2 (similar but using a type-equality superclass)
        class (F a ~ b) => C a b
    And try to sllve:
         [G] C a b
         [W] C a beta
    Follow the superclass rules to add
         [G] F a ~ b
         [D] F a ~ beta
    Now we get [D] beta ~ b, and can solve that.

    -- Example (tcfail138)
      class L a b | a -> b
      class (G a, L a b) => C a b

      instance C a b' => G (Maybe a)
      instance C a b  => C (Maybe a) a
      instance L (Maybe a) a

    When solving the superclasses of the (C (Maybe a) a) instance, we get
      [G] C a b, and hance by superclasses, [G] G a, [G] L a b
      [W] G (Maybe a)
    Use the instance decl to get
      [W] C a beta
    Generate its derived superclass
      [D] L a beta.  Now using fundeps, combine with [G] L a b to get
      [D] beta ~ b
    which is what we want.

Note [Danger of adding superclasses during solving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here's a serious, but now out-dated example, from #4497:

   class Num (RealOf t) => Normed t
   type family RealOf x

Assume the generated wanted constraint is:
   [W] RealOf e ~ e
   [W] Normed e

If we were to be adding the superclasses during simplification we'd get:
   [W] RealOf e ~ e
   [W] Normed e
   [D] RealOf e ~ fuv
   [D] Num fuv
==>
   e := fuv, Num fuv, Normed fuv, RealOf fuv ~ fuv

While looks exactly like our original constraint. If we add the
superclass of (Normed fuv) again we'd loop.  By adding superclasses
definitely only once, during canonicalisation, this situation can't
happen.

Mind you, now that Wanteds cannot rewrite Derived, I think this particular
situation can't happen.

Note [Nested quantified constraint superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (typecheck/should_compile/T17202)

  class C1 a
  class (forall c. C1 c) => C2 a
  class (forall b. (b ~ F a) => C2 a) => C3 a

Elsewhere in the code, we get a [G] g1 :: C3 a. We expand its superclass
to get [G] g2 :: (forall b. (b ~ F a) => C2 a). This constraint has a
superclass, as well. But we now must be careful: we cannot just add
(forall c. C1 c) as a Given, because we need to remember g2's context.
That new constraint is Given only when forall b. (b ~ F a) is true.

It's tempting to make the new Given be (forall b. (b ~ F a) => forall c. C1 c),
but that's problematic, because it's nested, and ForAllPred is not capable
of representing a nested quantified constraint. (We could change ForAllPred
to allow this, but the solution in this Note is much more local and simpler.)

So, we swizzle it around to get (forall b c. (b ~ F a) => C1 c).

More generally, if we are expanding the superclasses of
  g0 :: forall tvs. theta => cls tys
and find a superclass constraint
  forall sc_tvs. sc_theta => sc_inner_pred
we must have a selector
  sel_id :: forall cls_tvs. cls cls_tvs -> forall sc_tvs. sc_theta => sc_inner_pred
and thus build
  g_sc :: forall tvs sc_tvs. theta => sc_theta => sc_inner_pred
  g_sc = /\ tvs. /\ sc_tvs. \ theta_ids. \ sc_theta_ids.
         sel_id tys (g0 tvs theta_ids) sc_tvs sc_theta_ids

Actually, we cheat a bit by eta-reducing: note that sc_theta_ids are both the
last bound variables and the last arguments. This avoids the need to produce
the sc_theta_ids at all. So our final construction is

  g_sc = /\ tvs. /\ sc_tvs. \ theta_ids.
         sel_id tys (g0 tvs theta_ids) sc_tvs

  -}

makeSuperClasses :: [Ct] -> TcS [Ct]
-- Returns strict superclasses, transitively, see Note [The superclasses story]
-- See Note [The superclass story]
-- The loop-breaking here follows Note [Expanding superclasses] in GHC.Tc.Utils.TcType
-- Specifically, for an incoming (C t) constraint, we return all of (C t)'s
--    superclasses, up to /and including/ the first repetition of C
--
-- Example:  class D a => C a
--           class C [a] => D a
-- makeSuperClasses (C x) will return (D x, C [x])
--
-- NB: the incoming constraints have had their cc_pend_sc flag already
--     flipped to False, by isPendingScDict, so we are /obliged/ to at
--     least produce the immediate superclasses
makeSuperClasses :: [Ct] -> TcS [Ct]
makeSuperClasses [Ct]
cts = (Ct -> TcS [Ct]) -> [Ct] -> TcS [Ct]
forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b]
concatMapM Ct -> TcS [Ct]
go [Ct]
cts
  where
    go :: Ct -> TcS [Ct]
go (CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev, cc_class :: Ct -> Class
cc_class = Class
cls, cc_tyargs :: Ct -> [Xi]
cc_tyargs = [Xi]
tys })
      = CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [] [] Class
cls [Xi]
tys
    go (CQuantCan (QCI { qci_pred :: QCInst -> Xi
qci_pred = Xi
pred, qci_ev :: QCInst -> CtEvidence
qci_ev = CtEvidence
ev }))
      = ASSERT2( isClassPred pred, ppr pred )  -- The cts should all have
                                               -- class pred heads
        CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [TyVar]
tvs [Xi]
theta Class
cls [Xi]
tys
      where
        ([TyVar]
tvs, [Xi]
theta, Class
cls, [Xi]
tys) = Xi -> ([TyVar], [Xi], Class, [Xi])
tcSplitDFunTy (CtEvidence -> Xi
ctEvPred CtEvidence
ev)
    go Ct
ct = String -> SDoc -> TcS [Ct]
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"makeSuperClasses" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct)

mkStrictSuperClasses
    :: CtEvidence
    -> [TyVar] -> ThetaType  -- These two args are non-empty only when taking
                             -- superclasses of a /quantified/ constraint
    -> Class -> [Type] -> TcS [Ct]
-- Return constraints for the strict superclasses of
--   ev :: forall as. theta => cls tys
mkStrictSuperClasses :: CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [TyVar]
tvs [Xi]
theta Class
cls [Xi]
tys
  = NameSet
-> CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mk_strict_superclasses (Name -> NameSet
unitNameSet (Class -> Name
className Class
cls))
                           CtEvidence
ev [TyVar]
tvs [Xi]
theta Class
cls [Xi]
tys

mk_strict_superclasses :: NameSet -> CtEvidence
                       -> [TyVar] -> ThetaType
                       -> Class -> [Type] -> TcS [Ct]
-- Always return the immediate superclasses of (cls tys);
-- and expand their superclasses, provided none of them are in rec_clss
-- nor are repeated
mk_strict_superclasses :: NameSet
-> CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mk_strict_superclasses NameSet
rec_clss (CtGiven { ctev_evar :: CtEvidence -> TyVar
ctev_evar = TyVar
evar, ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc })
                       [TyVar]
tvs [Xi]
theta Class
cls [Xi]
tys
  = (TyVar -> TcS [Ct]) -> [TyVar] -> TcS [Ct]
forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b]
concatMapM (CtLoc -> TyVar -> TcS [Ct]
do_one_given (CtLoc -> CtLoc
mk_given_loc CtLoc
loc)) ([TyVar] -> TcS [Ct]) -> [TyVar] -> TcS [Ct]
forall a b. (a -> b) -> a -> b
$
    Class -> [TyVar]
classSCSelIds Class
cls
  where
    dict_ids :: [TyVar]
dict_ids  = [Xi] -> [TyVar]
mkTemplateLocals [Xi]
theta
    size :: TypeSize
size      = [Xi] -> TypeSize
sizeTypes [Xi]
tys

    do_one_given :: CtLoc -> TyVar -> TcS [Ct]
do_one_given CtLoc
given_loc TyVar
sel_id
      | HasDebugCallStack => Xi -> Bool
Xi -> Bool
isUnliftedType Xi
sc_pred
      , Bool -> Bool
not ([TyVar] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TyVar]
tvs Bool -> Bool -> Bool
&& [Xi] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Xi]
theta)
      = -- See Note [Equality superclasses in quantified constraints]
        [Ct] -> TcS [Ct]
forall (m :: * -> *) a. Monad m => a -> m a
return []
      | Bool
otherwise
      = do { CtEvidence
given_ev <- CtLoc -> (Xi, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
given_loc ((Xi, EvTerm) -> TcS CtEvidence) -> (Xi, EvTerm) -> TcS CtEvidence
forall a b. (a -> b) -> a -> b
$
                         TyVar -> Xi -> (Xi, EvTerm)
mk_given_desc TyVar
sel_id Xi
sc_pred
           ; NameSet -> CtEvidence -> [TyVar] -> [Xi] -> Xi -> TcS [Ct]
mk_superclasses NameSet
rec_clss CtEvidence
given_ev [TyVar]
tvs [Xi]
theta Xi
sc_pred }
      where
        sc_pred :: Xi
sc_pred = TyVar -> [Xi] -> Xi
classMethodInstTy TyVar
sel_id [Xi]
tys

      -- See Note [Nested quantified constraint superclasses]
    mk_given_desc :: Id -> PredType -> (PredType, EvTerm)
    mk_given_desc :: TyVar -> Xi -> (Xi, EvTerm)
mk_given_desc TyVar
sel_id Xi
sc_pred
      = (Xi
swizzled_pred, EvTerm
swizzled_evterm)
      where
        ([TyVar]
sc_tvs, Xi
sc_rho)          = Xi -> ([TyVar], Xi)
splitForAllTyCoVars Xi
sc_pred
        ([Scaled Xi]
sc_theta, Xi
sc_inner_pred) = Xi -> ([Scaled Xi], Xi)
splitFunTys Xi
sc_rho

        all_tvs :: [TyVar]
all_tvs       = [TyVar]
tvs [TyVar] -> [TyVar] -> [TyVar]
forall a. [a] -> [a] -> [a]
`chkAppend` [TyVar]
sc_tvs
        all_theta :: [Xi]
all_theta     = [Xi]
theta [Xi] -> [Xi] -> [Xi]
forall a. [a] -> [a] -> [a]
`chkAppend` ((Scaled Xi -> Xi) -> [Scaled Xi] -> [Xi]
forall a b. (a -> b) -> [a] -> [b]
map Scaled Xi -> Xi
forall a. Scaled a -> a
scaledThing [Scaled Xi]
sc_theta)
        swizzled_pred :: Xi
swizzled_pred = [TyVar] -> [Xi] -> Xi -> Xi
mkInfSigmaTy [TyVar]
all_tvs [Xi]
all_theta Xi
sc_inner_pred

        -- evar :: forall tvs. theta => cls tys
        -- sel_id :: forall cls_tvs. cls cls_tvs
        --                        -> forall sc_tvs. sc_theta => sc_inner_pred
        -- swizzled_evterm :: forall tvs sc_tvs. theta => sc_theta => sc_inner_pred
        swizzled_evterm :: EvTerm
swizzled_evterm = EvExpr -> EvTerm
EvExpr (EvExpr -> EvTerm) -> EvExpr -> EvTerm
forall a b. (a -> b) -> a -> b
$
          [TyVar] -> EvExpr -> EvExpr
forall b. [b] -> Expr b -> Expr b
mkLams [TyVar]
all_tvs (EvExpr -> EvExpr) -> EvExpr -> EvExpr
forall a b. (a -> b) -> a -> b
$
          [TyVar] -> EvExpr -> EvExpr
forall b. [b] -> Expr b -> Expr b
mkLams [TyVar]
dict_ids (EvExpr -> EvExpr) -> EvExpr -> EvExpr
forall a b. (a -> b) -> a -> b
$
          TyVar -> EvExpr
forall b. TyVar -> Expr b
Var TyVar
sel_id
            EvExpr -> [Xi] -> EvExpr
forall b. Expr b -> [Xi] -> Expr b
`mkTyApps` [Xi]
tys
            EvExpr -> EvExpr -> EvExpr
forall b. Expr b -> Expr b -> Expr b
`App` (TyVar -> EvExpr
evId TyVar
evar EvExpr -> [TyVar] -> EvExpr
forall b. Expr b -> [TyVar] -> Expr b
`mkVarApps` ([TyVar]
tvs [TyVar] -> [TyVar] -> [TyVar]
forall a. [a] -> [a] -> [a]
++ [TyVar]
dict_ids))
            EvExpr -> [TyVar] -> EvExpr
forall b. Expr b -> [TyVar] -> Expr b
`mkVarApps` [TyVar]
sc_tvs

    mk_given_loc :: CtLoc -> CtLoc
mk_given_loc CtLoc
loc
       | Class -> Bool
isCTupleClass Class
cls
       = CtLoc
loc   -- For tuple predicates, just take them apart, without
               -- adding their (large) size into the chain.  When we
               -- get down to a base predicate, we'll include its size.
               -- #10335

       | GivenOrigin SkolemInfo
skol_info <- CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc
         -- See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance
         -- for explantation of this transformation for givens
       = case SkolemInfo
skol_info of
            SkolemInfo
InstSkol -> CtLoc
loc { ctl_origin :: CtOrigin
ctl_origin = SkolemInfo -> CtOrigin
GivenOrigin (TypeSize -> SkolemInfo
InstSC TypeSize
size) }
            InstSC TypeSize
n -> CtLoc
loc { ctl_origin :: CtOrigin
ctl_origin = SkolemInfo -> CtOrigin
GivenOrigin (TypeSize -> SkolemInfo
InstSC (TypeSize
n TypeSize -> TypeSize -> TypeSize
forall a. Ord a => a -> a -> a
`max` TypeSize
size)) }
            SkolemInfo
_        -> CtLoc
loc

       | Bool
otherwise  -- Probably doesn't happen, since this function
       = CtLoc
loc        -- is only used for Givens, but does no harm

mk_strict_superclasses NameSet
rec_clss CtEvidence
ev [TyVar]
tvs [Xi]
theta Class
cls [Xi]
tys
  | (Xi -> Bool) -> [Xi] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all Xi -> Bool
noFreeVarsOfType [Xi]
tys
  = [Ct] -> TcS [Ct]
forall (m :: * -> *) a. Monad m => a -> m a
return [] -- Wanteds with no variables yield no deriveds.
              -- See Note [Improvement from Ground Wanteds]

  | Bool
otherwise -- Wanted/Derived case, just add Derived superclasses
              -- that can lead to improvement.
  = ASSERT2( null tvs && null theta, ppr tvs $$ ppr theta )
    (Xi -> TcS [Ct]) -> [Xi] -> TcS [Ct]
forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b]
concatMapM Xi -> TcS [Ct]
do_one_derived (Class -> [Xi] -> [Xi]
immSuperClasses Class
cls [Xi]
tys)
  where
    loc :: CtLoc
loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev

    do_one_derived :: Xi -> TcS [Ct]
do_one_derived Xi
sc_pred
      = do { CtEvidence
sc_ev <- CtLoc -> Xi -> TcS CtEvidence
newDerivedNC CtLoc
loc Xi
sc_pred
           ; NameSet -> CtEvidence -> [TyVar] -> [Xi] -> Xi -> TcS [Ct]
mk_superclasses NameSet
rec_clss CtEvidence
sc_ev [] [] Xi
sc_pred }

{- Note [Improvement from Ground Wanteds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose class C b a => D a b
and consider
  [W] D Int Bool
Is there any point in emitting [D] C Bool Int?  No!  The only point of
emitting superclass constraints for W/D constraints is to get
improvement, extra unifications that result from functional
dependencies.  See Note [Why adding superclasses can help] above.

But no variables means no improvement; case closed.
-}

mk_superclasses :: NameSet -> CtEvidence
                -> [TyVar] -> ThetaType -> PredType -> TcS [Ct]
-- Return this constraint, plus its superclasses, if any
mk_superclasses :: NameSet -> CtEvidence -> [TyVar] -> [Xi] -> Xi -> TcS [Ct]
mk_superclasses NameSet
rec_clss CtEvidence
ev [TyVar]
tvs [Xi]
theta Xi
pred
  | ClassPred Class
cls [Xi]
tys <- Xi -> Pred
classifyPredType Xi
pred
  = NameSet
-> CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mk_superclasses_of NameSet
rec_clss CtEvidence
ev [TyVar]
tvs [Xi]
theta Class
cls [Xi]
tys

  | Bool
otherwise   -- Superclass is not a class predicate
  = [Ct] -> TcS [Ct]
forall (m :: * -> *) a. Monad m => a -> m a
return [CtEvidence -> Ct
mkNonCanonical CtEvidence
ev]

mk_superclasses_of :: NameSet -> CtEvidence
                   -> [TyVar] -> ThetaType -> Class -> [Type]
                   -> TcS [Ct]
-- Always return this class constraint,
-- and expand its superclasses
mk_superclasses_of :: NameSet
-> CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mk_superclasses_of NameSet
rec_clss CtEvidence
ev [TyVar]
tvs [Xi]
theta Class
cls [Xi]
tys
  | Bool
loop_found = do { String -> SDoc -> TcS ()
traceTcS String
"mk_superclasses_of: loop" (Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
<+> [Xi] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Xi]
tys)
                    ; [Ct] -> TcS [Ct]
forall (m :: * -> *) a. Monad m => a -> m a
return [Ct
this_ct] }  -- cc_pend_sc of this_ct = True
  | Bool
otherwise  = do { String -> SDoc -> TcS ()
traceTcS String
"mk_superclasses_of" ([SDoc] -> SDoc
vcat [ Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
cls SDoc -> SDoc -> SDoc
<+> [Xi] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Xi]
tys
                                                          , Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Class -> Bool
isCTupleClass Class
cls)
                                                          , NameSet -> SDoc
forall a. Outputable a => a -> SDoc
ppr NameSet
rec_clss
                                                          ])
                    ; [Ct]
sc_cts <- NameSet
-> CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mk_strict_superclasses NameSet
rec_clss' CtEvidence
ev [TyVar]
tvs [Xi]
theta Class
cls [Xi]
tys
                    ; [Ct] -> TcS [Ct]
forall (m :: * -> *) a. Monad m => a -> m a
return (Ct
this_ct Ct -> [Ct] -> [Ct]
forall a. a -> [a] -> [a]
: [Ct]
sc_cts) }
                                   -- cc_pend_sc of this_ct = False
  where
    cls_nm :: Name
cls_nm     = Class -> Name
className Class
cls
    loop_found :: Bool
loop_found = Bool -> Bool
not (Class -> Bool
isCTupleClass Class
cls) Bool -> Bool -> Bool
&& Name
cls_nm Name -> NameSet -> Bool
`elemNameSet` NameSet
rec_clss
                 -- Tuples never contribute to recursion, and can be nested
    rec_clss' :: NameSet
rec_clss'  = NameSet
rec_clss NameSet -> Name -> NameSet
`extendNameSet` Name
cls_nm

    this_ct :: Ct
this_ct | [TyVar] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TyVar]
tvs, [Xi] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Xi]
theta
            = CDictCan :: CtEvidence -> Class -> [Xi] -> Bool -> Ct
CDictCan { cc_ev :: CtEvidence
cc_ev = CtEvidence
ev, cc_class :: Class
cc_class = Class
cls, cc_tyargs :: [Xi]
cc_tyargs = [Xi]
tys
                       , cc_pend_sc :: Bool
cc_pend_sc = Bool
loop_found }
                 -- NB: If there is a loop, we cut off, so we have not
                 --     added the superclasses, hence cc_pend_sc = True
            | Bool
otherwise
            = QCInst -> Ct
CQuantCan (QCI :: CtEvidence -> [TyVar] -> Xi -> Bool -> QCInst
QCI { qci_tvs :: [TyVar]
qci_tvs = [TyVar]
tvs, qci_pred :: Xi
qci_pred = Class -> [Xi] -> Xi
mkClassPred Class
cls [Xi]
tys
                             , qci_ev :: CtEvidence
qci_ev = CtEvidence
ev
                             , qci_pend_sc :: Bool
qci_pend_sc = Bool
loop_found })


{- Note [Equality superclasses in quantified constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (#15359, #15593, #15625)
  f :: (forall a. theta => a ~ b) => stuff

It's a bit odd to have a local, quantified constraint for `(a~b)`,
but some people want such a thing (see the tickets). And for
Coercible it is definitely useful
  f :: forall m. (forall p q. Coercible p q => Coercible (m p) (m q)))
                 => stuff

Moreover it's not hard to arrange; we just need to look up /equality/
constraints in the quantified-constraint environment, which we do in
GHC.Tc.Solver.Interact.doTopReactOther.

There is a wrinkle though, in the case where 'theta' is empty, so
we have
  f :: (forall a. a~b) => stuff

Now, potentially, the superclass machinery kicks in, in
makeSuperClasses, giving us a a second quantified constraint
       (forall a. a ~# b)
BUT this is an unboxed value!  And nothing has prepared us for
dictionary "functions" that are unboxed.  Actually it does just
about work, but the simplifier ends up with stuff like
   case (/\a. eq_sel d) of df -> ...(df @Int)...
and fails to simplify that any further.  And it doesn't satisfy
isPredTy any more.

So for now we simply decline to take superclasses in the quantified
case.  Instead we have a special case in GHC.Tc.Solver.Interact.doTopReactOther,
which looks for primitive equalities specially in the quantified
constraints.

See also Note [Evidence for quantified constraints] in GHC.Core.Predicate.


************************************************************************
*                                                                      *
*                      Irreducibles canonicalization
*                                                                      *
************************************************************************
-}

canIrred :: CtEvidence -> TcS (StopOrContinue Ct)
-- Precondition: ty not a tuple and no other evidence form
canIrred :: CtEvidence -> TcS (StopOrContinue Ct)
canIrred CtEvidence
ev
  = do { let pred :: Xi
pred = CtEvidence -> Xi
ctEvPred CtEvidence
ev
       ; String -> SDoc -> TcS ()
traceTcS String
"can_pred" (String -> SDoc
text String
"IrredPred = " SDoc -> SDoc -> SDoc
<+> Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
pred)
       ; (Xi
xi,TcCoercion
co) <- CtEvidence -> Xi -> TcS (Xi, TcCoercion)
rewrite CtEvidence
ev Xi
pred -- co :: xi ~ pred
       ; CtEvidence -> Xi -> TcCoercion -> TcS (StopOrContinue CtEvidence)
rewriteEvidence CtEvidence
ev Xi
xi TcCoercion
co TcS (StopOrContinue CtEvidence)
-> (CtEvidence -> TcS (StopOrContinue Ct))
-> TcS (StopOrContinue Ct)
forall a b.
TcS (StopOrContinue a)
-> (a -> TcS (StopOrContinue b)) -> TcS (StopOrContinue b)
`andWhenContinue` \ CtEvidence
new_ev ->

    do { -- Re-classify, in case rewriting has improved its shape
         -- Code is like the canNC, except
         -- that the IrredPred branch stops work
       ; case Xi -> Pred
classifyPredType (CtEvidence -> Xi
ctEvPred CtEvidence
new_ev) of
           ClassPred Class
cls [Xi]
tys     -> CtEvidence -> Class -> [Xi] -> TcS (StopOrContinue Ct)
canClassNC CtEvidence
new_ev Class
cls [Xi]
tys
           EqPred EqRel
eq_rel Xi
ty1 Xi
ty2 -> CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqNC CtEvidence
new_ev EqRel
eq_rel Xi
ty1 Xi
ty2
           ForAllPred [TyVar]
tvs [Xi]
th Xi
p   -> -- this is highly suspect; Quick Look
                                    -- should never leave a meta-var filled
                                    -- in with a polytype. This is #18987.
                                    do String -> SDoc -> TcS ()
traceTcS String
"canEvNC:forall" (Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
pred)
                                       CtEvidence -> [TyVar] -> [Xi] -> Xi -> TcS (StopOrContinue Ct)
canForAllNC CtEvidence
ev [TyVar]
tvs [Xi]
th Xi
p
           IrredPred {}          -> Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (Ct -> TcS (StopOrContinue Ct)) -> Ct -> TcS (StopOrContinue Ct)
forall a b. (a -> b) -> a -> b
$
                                    CtIrredStatus -> CtEvidence -> Ct
mkIrredCt CtIrredStatus
OtherCIS CtEvidence
new_ev } }

{- *********************************************************************
*                                                                      *
*                      Quantified predicates
*                                                                      *
********************************************************************* -}

{- Note [Quantified constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The -XQuantifiedConstraints extension allows type-class contexts like this:

  data Rose f x = Rose x (f (Rose f x))

  instance (Eq a, forall b. Eq b => Eq (f b))
        => Eq (Rose f a)  where
    (Rose x1 rs1) == (Rose x2 rs2) = x1==x2 && rs1 == rs2

Note the (forall b. Eq b => Eq (f b)) in the instance contexts.
This quantified constraint is needed to solve the
 [W] (Eq (f (Rose f x)))
constraint which arises form the (==) definition.

The wiki page is
  https://gitlab.haskell.org/ghc/ghc/wikis/quantified-constraints
which in turn contains a link to the GHC Proposal where the change
is specified, and a Haskell Symposium paper about it.

We implement two main extensions to the design in the paper:

 1. We allow a variable in the instance head, e.g.
      f :: forall m a. (forall b. m b) => D (m a)
    Notice the 'm' in the head of the quantified constraint, not
    a class.

 2. We support superclasses to quantified constraints.
    For example (contrived):
      f :: (Ord b, forall b. Ord b => Ord (m b)) => m a -> m a -> Bool
      f x y = x==y
    Here we need (Eq (m a)); but the quantified constraint deals only
    with Ord.  But we can make it work by using its superclass.

Here are the moving parts
  * Language extension {-# LANGUAGE QuantifiedConstraints #-}
    and add it to ghc-boot-th:GHC.LanguageExtensions.Type.Extension

  * A new form of evidence, EvDFun, that is used to discharge
    such wanted constraints

  * checkValidType gets some changes to accept forall-constraints
    only in the right places.

  * Predicate.Pred gets a new constructor ForAllPred, and
    and classifyPredType analyses a PredType to decompose
    the new forall-constraints

  * GHC.Tc.Solver.Monad.InertCans gets an extra field, inert_insts,
    which holds all the Given forall-constraints.  In effect,
    such Given constraints are like local instance decls.

  * When trying to solve a class constraint, via
    GHC.Tc.Solver.Interact.matchInstEnv, use the InstEnv from inert_insts
    so that we include the local Given forall-constraints
    in the lookup.  (See GHC.Tc.Solver.Monad.getInstEnvs.)

  * GHC.Tc.Solver.Canonical.canForAll deals with solving a
    forall-constraint.  See
       Note [Solving a Wanted forall-constraint]

  * We augment the kick-out code to kick out an inert
    forall constraint if it can be rewritten by a new
    type equality; see GHC.Tc.Solver.Monad.kick_out_rewritable

Note that a quantified constraint is never /inferred/
(by GHC.Tc.Solver.simplifyInfer).  A function can only have a
quantified constraint in its type if it is given an explicit
type signature.

-}

canForAllNC :: CtEvidence -> [TyVar] -> TcThetaType -> TcPredType
            -> TcS (StopOrContinue Ct)
canForAllNC :: CtEvidence -> [TyVar] -> [Xi] -> Xi -> TcS (StopOrContinue Ct)
canForAllNC CtEvidence
ev [TyVar]
tvs [Xi]
theta Xi
pred
  | CtEvidence -> Bool
isGiven CtEvidence
ev  -- See Note [Eagerly expand given superclasses]
  , Just (Class
cls, [Xi]
tys) <- Maybe (Class, [Xi])
cls_pred_tys_maybe
  = do { [Ct]
sc_cts <- CtEvidence -> [TyVar] -> [Xi] -> Class -> [Xi] -> TcS [Ct]
mkStrictSuperClasses CtEvidence
ev [TyVar]
tvs [Xi]
theta Class
cls [Xi]
tys
       ; [Ct] -> TcS ()
emitWork [Ct]
sc_cts
       ; CtEvidence -> Bool -> TcS (StopOrContinue Ct)
canForAll CtEvidence
ev Bool
False }

  | Bool
otherwise
  = CtEvidence -> Bool -> TcS (StopOrContinue Ct)
canForAll CtEvidence
ev (Maybe (Class, [Xi]) -> Bool
forall a. Maybe a -> Bool
isJust Maybe (Class, [Xi])
cls_pred_tys_maybe)

  where
    cls_pred_tys_maybe :: Maybe (Class, [Xi])
cls_pred_tys_maybe = Xi -> Maybe (Class, [Xi])
getClassPredTys_maybe Xi
pred

canForAll :: CtEvidence -> Bool -> TcS (StopOrContinue Ct)
-- We have a constraint (forall as. blah => C tys)
canForAll :: CtEvidence -> Bool -> TcS (StopOrContinue Ct)
canForAll CtEvidence
ev Bool
pend_sc
  = do { -- First rewrite it to apply the current substitution
         let pred :: Xi
pred = CtEvidence -> Xi
ctEvPred CtEvidence
ev
       ; (Xi
xi,TcCoercion
co) <- CtEvidence -> Xi -> TcS (Xi, TcCoercion)
rewrite CtEvidence
ev Xi
pred -- co :: xi ~ pred
       ; CtEvidence -> Xi -> TcCoercion -> TcS (StopOrContinue CtEvidence)
rewriteEvidence CtEvidence
ev Xi
xi TcCoercion
co TcS (StopOrContinue CtEvidence)
-> (CtEvidence -> TcS (StopOrContinue Ct))
-> TcS (StopOrContinue Ct)
forall a b.
TcS (StopOrContinue a)
-> (a -> TcS (StopOrContinue b)) -> TcS (StopOrContinue b)
`andWhenContinue` \ CtEvidence
new_ev ->

    do { -- Now decompose into its pieces and solve it
         -- (It takes a lot less code to rewrite before decomposing.)
       ; case Xi -> Pred
classifyPredType (CtEvidence -> Xi
ctEvPred CtEvidence
new_ev) of
           ForAllPred [TyVar]
tvs [Xi]
theta Xi
pred
              -> CtEvidence
-> [TyVar] -> [Xi] -> Xi -> Bool -> TcS (StopOrContinue Ct)
solveForAll CtEvidence
new_ev [TyVar]
tvs [Xi]
theta Xi
pred Bool
pend_sc
           Pred
_  -> String -> SDoc -> TcS (StopOrContinue Ct)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"canForAll" (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
new_ev)
    } }

solveForAll :: CtEvidence -> [TyVar] -> TcThetaType -> PredType -> Bool
            -> TcS (StopOrContinue Ct)
solveForAll :: CtEvidence
-> [TyVar] -> [Xi] -> Xi -> Bool -> TcS (StopOrContinue Ct)
solveForAll CtEvidence
ev [TyVar]
tvs [Xi]
theta Xi
pred Bool
pend_sc
  | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest } <- CtEvidence
ev
  = -- See Note [Solving a Wanted forall-constraint]
    do { let skol_info :: SkolemInfo
skol_info = SkolemInfo
QuantCtxtSkol
             empty_subst :: TCvSubst
empty_subst = InScopeSet -> TCvSubst
mkEmptyTCvSubst (InScopeSet -> TCvSubst) -> InScopeSet -> TCvSubst
forall a b. (a -> b) -> a -> b
$ VarSet -> InScopeSet
mkInScopeSet (VarSet -> InScopeSet) -> VarSet -> InScopeSet
forall a b. (a -> b) -> a -> b
$
                           [Xi] -> VarSet
tyCoVarsOfTypes (Xi
predXi -> [Xi] -> [Xi]
forall a. a -> [a] -> [a]
:[Xi]
theta) VarSet -> [TyVar] -> VarSet
`delVarSetList` [TyVar]
tvs
       ; (TCvSubst
subst, [TyVar]
skol_tvs) <- TCvSubst -> [TyVar] -> TcS (TCvSubst, [TyVar])
tcInstSkolTyVarsX TCvSubst
empty_subst [TyVar]
tvs
       ; [TyVar]
given_ev_vars <- (Xi -> TcS TyVar) -> [Xi] -> TcS [TyVar]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM Xi -> TcS TyVar
newEvVar (HasCallStack => TCvSubst -> [Xi] -> [Xi]
TCvSubst -> [Xi] -> [Xi]
substTheta TCvSubst
subst [Xi]
theta)

       ; (TcLevel
lvl, (TyVar
w_id, Bag Ct
wanteds))
             <- SDoc -> TcS (TyVar, Bag Ct) -> TcS (TcLevel, (TyVar, Bag Ct))
forall a. SDoc -> TcS a -> TcS (TcLevel, a)
pushLevelNoWorkList (SkolemInfo -> SDoc
forall a. Outputable a => a -> SDoc
ppr SkolemInfo
skol_info) (TcS (TyVar, Bag Ct) -> TcS (TcLevel, (TyVar, Bag Ct)))
-> TcS (TyVar, Bag Ct) -> TcS (TcLevel, (TyVar, Bag Ct))
forall a b. (a -> b) -> a -> b
$
                do { CtEvidence
wanted_ev <- CtLoc -> Xi -> TcS CtEvidence
newWantedEvVarNC CtLoc
loc (Xi -> TcS CtEvidence) -> Xi -> TcS CtEvidence
forall a b. (a -> b) -> a -> b
$
                                  HasCallStack => TCvSubst -> Xi -> Xi
TCvSubst -> Xi -> Xi
substTy TCvSubst
subst Xi
pred
                   ; (TyVar, Bag Ct) -> TcS (TyVar, Bag Ct)
forall (m :: * -> *) a. Monad m => a -> m a
return ( CtEvidence -> TyVar
ctEvEvId CtEvidence
wanted_ev
                            , Ct -> Bag Ct
forall a. a -> Bag a
unitBag (CtEvidence -> Ct
mkNonCanonical CtEvidence
wanted_ev)) }

      ; TcEvBinds
ev_binds <- TcLevel
-> SkolemInfo -> [TyVar] -> [TyVar] -> Bag Ct -> TcS TcEvBinds
emitImplicationTcS TcLevel
lvl SkolemInfo
skol_info [TyVar]
skol_tvs
                                       [TyVar]
given_ev_vars Bag Ct
wanteds

      ; TcEvDest -> EvTerm -> TcS ()
setWantedEvTerm TcEvDest
dest (EvTerm -> TcS ()) -> EvTerm -> TcS ()
forall a b. (a -> b) -> a -> b
$
        EvFun :: [TyVar] -> [TyVar] -> TcEvBinds -> TyVar -> EvTerm
EvFun { et_tvs :: [TyVar]
et_tvs = [TyVar]
skol_tvs, et_given :: [TyVar]
et_given = [TyVar]
given_ev_vars
              , et_binds :: TcEvBinds
et_binds = TcEvBinds
ev_binds, et_body :: TyVar
et_body = TyVar
w_id }

      ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Wanted forall-constraint" }

  | CtEvidence -> Bool
isGiven CtEvidence
ev   -- See Note [Solving a Given forall-constraint]
  = do { QCInst -> TcS ()
addInertForAll QCInst
qci
       ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Given forall-constraint" }

  | Bool
otherwise
  = do { String -> SDoc -> TcS ()
traceTcS String
"discarding derived forall-constraint" (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev)
       ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Derived forall-constraint" }
  where
    loc :: CtLoc
loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    qci :: QCInst
qci = QCI :: CtEvidence -> [TyVar] -> Xi -> Bool -> QCInst
QCI { qci_ev :: CtEvidence
qci_ev = CtEvidence
ev, qci_tvs :: [TyVar]
qci_tvs = [TyVar]
tvs
              , qci_pred :: Xi
qci_pred = Xi
pred, qci_pend_sc :: Bool
qci_pend_sc = Bool
pend_sc }

{- Note [Solving a Wanted forall-constraint]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Solving a wanted forall (quantified) constraint
  [W] df :: forall ab. (Eq a, Ord b) => C x a b
is delightfully easy.   Just build an implication constraint
    forall ab. (g1::Eq a, g2::Ord b) => [W] d :: C x a
and discharge df thus:
    df = /\ab. \g1 g2. let <binds> in d
where <binds> is filled in by solving the implication constraint.
All the machinery is to hand; there is little to do.

Note [Solving a Given forall-constraint]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For a Given constraint
  [G] df :: forall ab. (Eq a, Ord b) => C x a b
we just add it to TcS's local InstEnv of known instances,
via addInertForall.  Then, if we look up (C x Int Bool), say,
we'll find a match in the InstEnv.


************************************************************************
*                                                                      *
*        Equalities
*                                                                      *
************************************************************************

Note [Canonicalising equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to canonicalise an equality, we look at the structure of the
two types at hand, looking for similarities. A difficulty is that the
types may look dissimilar before rewriting but similar after rewriting.
However, we don't just want to jump in and rewrite right away, because
this might be wasted effort. So, after looking for similarities and failing,
we rewrite and then try again. Of course, we don't want to loop, so we
track whether or not we've already rewritten.

It is conceivable to do a better job at tracking whether or not a type
is rewritten, but this is left as future work. (Mar '15)


Note [Decomposing FunTy]
~~~~~~~~~~~~~~~~~~~~~~~~
can_eq_nc' may attempt to decompose a FunTy that is un-zonked.  This
means that we may very well have a FunTy containing a type of some
unknown kind. For instance, we may have,

    FunTy (a :: k) Int

Where k is a unification variable. So the calls to getRuntimeRep_maybe may
fail (returning Nothing).  In that case we'll fall through, zonk, and try again.
Zonking should fill the variable k, meaning that decomposition will succeed the
second time around.

Also note that we require the AnonArgFlag to match.  This will stop
us decomposing
   (Int -> Bool)  ~  (Show a => blah)
It's as if we treat (->) and (=>) as different type constructors.
-}

canEqNC :: CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqNC :: CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqNC CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ty2
  = do { Either (Pair Xi) Xi
result <- Xi -> Xi -> TcS (Either (Pair Xi) Xi)
zonk_eq_types Xi
ty1 Xi
ty2
       ; case Either (Pair Xi) Xi
result of
           Left (Pair Xi
ty1' Xi
ty2') -> Bool
-> CtEvidence
-> EqRel
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_nc Bool
False CtEvidence
ev EqRel
eq_rel Xi
ty1' Xi
ty1 Xi
ty2' Xi
ty2
           Right Xi
ty              -> CtEvidence -> EqRel -> Xi -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
eq_rel Xi
ty }

can_eq_nc
   :: Bool            -- True => both types are rewritten
   -> CtEvidence
   -> EqRel
   -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
   -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
   -> TcS (StopOrContinue Ct)
can_eq_nc :: Bool
-> CtEvidence
-> EqRel
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_nc Bool
rewritten CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ps_ty1 Xi
ty2 Xi
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_nc" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr Bool
rewritten, CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev, EqRel -> SDoc
forall a. Outputable a => a -> SDoc
ppr EqRel
eq_rel, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty1, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ps_ty1, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty2, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ps_ty2 ]
       ; GlobalRdrEnv
rdr_env <- TcS GlobalRdrEnv
getGlobalRdrEnvTcS
       ; (FamInstEnv, FamInstEnv)
fam_insts <- TcS (FamInstEnv, FamInstEnv)
getFamInstEnvs
       ; Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
fam_insts CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ps_ty1 Xi
ty2 Xi
ps_ty2 }

can_eq_nc'
   :: Bool           -- True => both input types are rewritten
   -> GlobalRdrEnv   -- needed to see which newtypes are in scope
   -> FamInstEnvs    -- needed to unwrap data instances
   -> CtEvidence
   -> EqRel
   -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
   -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
   -> TcS (StopOrContinue Ct)

-- See Note [Comparing nullary type synonyms] in GHC.Core.Type.
can_eq_nc' :: Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
_flat GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel ty1 :: Xi
ty1@(TyConApp TyCon
tc1 []) Xi
_ps_ty1 (TyConApp TyCon
tc2 []) Xi
_ps_ty2
  | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2
  = CtEvidence -> EqRel -> Xi -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
eq_rel Xi
ty1

-- Expand synonyms first; see Note [Type synonyms and canonicalization]
can_eq_nc' Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ps_ty1 Xi
ty2 Xi
ps_ty2
  | Just Xi
ty1' <- Xi -> Maybe Xi
tcView Xi
ty1 = Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Xi
ty1' Xi
ps_ty1 Xi
ty2  Xi
ps_ty2
  | Just Xi
ty2' <- Xi -> Maybe Xi
tcView Xi
ty2 = Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Xi
ty1  Xi
ps_ty1 Xi
ty2' Xi
ps_ty2

-- need to check for reflexivity in the ReprEq case.
-- See Note [Eager reflexivity check]
-- Check only when rewritten because the zonk_eq_types check in canEqNC takes
-- care of the non-rewritten case.
can_eq_nc' Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
ReprEq Xi
ty1 Xi
_ Xi
ty2 Xi
_
  | Xi
ty1 HasDebugCallStack => Xi -> Xi -> Bool
Xi -> Xi -> Bool
`tcEqType` Xi
ty2
  = CtEvidence -> EqRel -> Xi -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
ReprEq Xi
ty1

-- When working with ReprEq, unwrap newtypes.
-- See Note [Unwrap newtypes first]
-- This must be above the TyVarTy case, in order to guarantee (TyEq:N)
can_eq_nc' Bool
_rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ps_ty1 Xi
ty2 Xi
ps_ty2
  | EqRel
ReprEq <- EqRel
eq_rel
  , Just ((Bag GlobalRdrElt, TcCoercion), Xi)
stuff1 <- (FamInstEnv, FamInstEnv)
-> GlobalRdrEnv -> Xi -> Maybe ((Bag GlobalRdrElt, TcCoercion), Xi)
tcTopNormaliseNewTypeTF_maybe (FamInstEnv, FamInstEnv)
envs GlobalRdrEnv
rdr_env Xi
ty1
  = CtEvidence
-> SwapFlag
-> Xi
-> ((Bag GlobalRdrElt, TcCoercion), Xi)
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_newtype_nc CtEvidence
ev SwapFlag
NotSwapped Xi
ty1 ((Bag GlobalRdrElt, TcCoercion), Xi)
stuff1 Xi
ty2 Xi
ps_ty2

  | EqRel
ReprEq <- EqRel
eq_rel
  , Just ((Bag GlobalRdrElt, TcCoercion), Xi)
stuff2 <- (FamInstEnv, FamInstEnv)
-> GlobalRdrEnv -> Xi -> Maybe ((Bag GlobalRdrElt, TcCoercion), Xi)
tcTopNormaliseNewTypeTF_maybe (FamInstEnv, FamInstEnv)
envs GlobalRdrEnv
rdr_env Xi
ty2
  = CtEvidence
-> SwapFlag
-> Xi
-> ((Bag GlobalRdrElt, TcCoercion), Xi)
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_newtype_nc CtEvidence
ev SwapFlag
IsSwapped  Xi
ty2 ((Bag GlobalRdrElt, TcCoercion), Xi)
stuff2 Xi
ty1 Xi
ps_ty1

-- Then, get rid of casts
can_eq_nc' Bool
rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel (CastTy Xi
ty1 TcCoercion
co1) Xi
_ Xi
ty2 Xi
ps_ty2
  | Maybe CanEqLHS -> Bool
forall a. Maybe a -> Bool
isNothing (Xi -> Maybe CanEqLHS
canEqLHS_maybe Xi
ty2)  -- See (3) in Note [Equalities with incompatible kinds]
  = Bool
-> CtEvidence
-> EqRel
-> SwapFlag
-> Xi
-> TcCoercion
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCast Bool
rewritten CtEvidence
ev EqRel
eq_rel SwapFlag
NotSwapped Xi
ty1 TcCoercion
co1 Xi
ty2 Xi
ps_ty2
can_eq_nc' Bool
rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ps_ty1 (CastTy Xi
ty2 TcCoercion
co2) Xi
_
  | Maybe CanEqLHS -> Bool
forall a. Maybe a -> Bool
isNothing (Xi -> Maybe CanEqLHS
canEqLHS_maybe Xi
ty1)  -- See (3) in Note [Equalities with incompatible kinds]
  = Bool
-> CtEvidence
-> EqRel
-> SwapFlag
-> Xi
-> TcCoercion
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCast Bool
rewritten CtEvidence
ev EqRel
eq_rel SwapFlag
IsSwapped Xi
ty2 TcCoercion
co2 Xi
ty1 Xi
ps_ty1

----------------------
-- Otherwise try to decompose
----------------------

-- Literals
can_eq_nc' Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel ty1 :: Xi
ty1@(LitTy TyLit
l1) Xi
_ (LitTy TyLit
l2) Xi
_
 | TyLit
l1 TyLit -> TyLit -> Bool
forall a. Eq a => a -> a -> Bool
== TyLit
l2
  = do { CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev (TcCoercion -> EvTerm
evCoercion (TcCoercion -> EvTerm) -> TcCoercion -> EvTerm
forall a b. (a -> b) -> a -> b
$ Role -> Xi -> TcCoercion
mkReflCo (EqRel -> Role
eqRelRole EqRel
eq_rel) Xi
ty1)
       ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Equal LitTy" }

-- Decompose FunTy: (s -> t) and (c => t)
-- NB: don't decompose (Int -> blah) ~ (Show a => blah)
can_eq_nc' Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel
           (FunTy { ft_mult :: Xi -> Xi
ft_mult = Xi
am1, ft_af :: Xi -> AnonArgFlag
ft_af = AnonArgFlag
af1, ft_arg :: Xi -> Xi
ft_arg = Xi
ty1a, ft_res :: Xi -> Xi
ft_res = Xi
ty1b }) Xi
_ps_ty1
           (FunTy { ft_mult :: Xi -> Xi
ft_mult = Xi
am2, ft_af :: Xi -> AnonArgFlag
ft_af = AnonArgFlag
af2, ft_arg :: Xi -> Xi
ft_arg = Xi
ty2a, ft_res :: Xi -> Xi
ft_res = Xi
ty2b }) Xi
_ps_ty2
  | AnonArgFlag
af1 AnonArgFlag -> AnonArgFlag -> Bool
forall a. Eq a => a -> a -> Bool
== AnonArgFlag
af2   -- Don't decompose (Int -> blah) ~ (Show a => blah)
  , Just Xi
ty1a_rep <- HasDebugCallStack => Xi -> Maybe Xi
Xi -> Maybe Xi
getRuntimeRep_maybe Xi
ty1a  -- getRutimeRep_maybe:
  , Just Xi
ty1b_rep <- HasDebugCallStack => Xi -> Maybe Xi
Xi -> Maybe Xi
getRuntimeRep_maybe Xi
ty1b  -- see Note [Decomposing FunTy]
  , Just Xi
ty2a_rep <- HasDebugCallStack => Xi -> Maybe Xi
Xi -> Maybe Xi
getRuntimeRep_maybe Xi
ty2a
  , Just Xi
ty2b_rep <- HasDebugCallStack => Xi -> Maybe Xi
Xi -> Maybe Xi
getRuntimeRep_maybe Xi
ty2b
  = CtEvidence
-> EqRel -> TyCon -> [Xi] -> [Xi] -> TcS (StopOrContinue Ct)
canDecomposableTyConAppOK CtEvidence
ev EqRel
eq_rel TyCon
funTyCon
                              [Xi
am1, Xi
ty1a_rep, Xi
ty1b_rep, Xi
ty1a, Xi
ty1b]
                              [Xi
am2, Xi
ty2a_rep, Xi
ty2b_rep, Xi
ty2a, Xi
ty2b]

-- Decompose type constructor applications
-- NB: we have expanded type synonyms already
can_eq_nc' Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
_ Xi
ty2 Xi
_
  | Just (TyCon
tc1, [Xi]
tys1) <- HasCallStack => Xi -> Maybe (TyCon, [Xi])
Xi -> Maybe (TyCon, [Xi])
tcSplitTyConApp_maybe Xi
ty1
  , Just (TyCon
tc2, [Xi]
tys2) <- HasCallStack => Xi -> Maybe (TyCon, [Xi])
Xi -> Maybe (TyCon, [Xi])
tcSplitTyConApp_maybe Xi
ty2
   -- we want to catch e.g. Maybe Int ~ (Int -> Int) here for better
   -- error messages rather than decomposing into AppTys;
   -- hence no direct match on TyConApp
  , Bool -> Bool
not (TyCon -> Bool
isTypeFamilyTyCon TyCon
tc1)
  , Bool -> Bool
not (TyCon -> Bool
isTypeFamilyTyCon TyCon
tc2)
  = CtEvidence
-> EqRel
-> TyCon
-> [Xi]
-> TyCon
-> [Xi]
-> TcS (StopOrContinue Ct)
canTyConApp CtEvidence
ev EqRel
eq_rel TyCon
tc1 [Xi]
tys1 TyCon
tc2 [Xi]
tys2

can_eq_nc' Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel
           s1 :: Xi
s1@(ForAllTy (Bndr TyVar
_ ArgFlag
vis1) Xi
_) Xi
_
           s2 :: Xi
s2@(ForAllTy (Bndr TyVar
_ ArgFlag
vis2) Xi
_) Xi
_
  | ArgFlag
vis1 ArgFlag -> ArgFlag -> Bool
`sameVis` ArgFlag
vis2 -- Note [ForAllTy and typechecker equality]
  = CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
can_eq_nc_forall CtEvidence
ev EqRel
eq_rel Xi
s1 Xi
s2

-- See Note [Canonicalising type applications] about why we require rewritten types
-- Use tcSplitAppTy, not matching on AppTy, to catch oversaturated type families
-- NB: Only decompose AppTy for nominal equality. See Note [Decomposing equality]
can_eq_nc' Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
NomEq Xi
ty1 Xi
_ Xi
ty2 Xi
_
  | Just (Xi
t1, Xi
s1) <- Xi -> Maybe (Xi, Xi)
tcSplitAppTy_maybe Xi
ty1
  , Just (Xi
t2, Xi
s2) <- Xi -> Maybe (Xi, Xi)
tcSplitAppTy_maybe Xi
ty2
  = CtEvidence -> Xi -> Xi -> Xi -> Xi -> TcS (StopOrContinue Ct)
can_eq_app CtEvidence
ev Xi
t1 Xi
s1 Xi
t2 Xi
s2

-------------------
-- Can't decompose.
-------------------

-- No similarity in type structure detected. Rewrite and try again.
can_eq_nc' Bool
False GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Xi
_ Xi
ps_ty1 Xi
_ Xi
ps_ty2
  = do { (Xi
xi1, TcCoercion
co1) <- CtEvidence -> Xi -> TcS (Xi, TcCoercion)
rewrite CtEvidence
ev Xi
ps_ty1
       ; (Xi
xi2, TcCoercion
co2) <- CtEvidence -> Xi -> TcS (Xi, TcCoercion)
rewrite CtEvidence
ev Xi
ps_ty2
       ; CtEvidence
new_ev <- CtEvidence
-> SwapFlag
-> Xi
-> Xi
-> TcCoercion
-> TcCoercion
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
NotSwapped Xi
xi1 Xi
xi2 TcCoercion
co1 TcCoercion
co2
       ; Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_nc' Bool
True GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
new_ev EqRel
eq_rel Xi
xi1 Xi
xi1 Xi
xi2 Xi
xi2 }

----------------------------
-- Look for a canonical LHS. See Note [Canonical LHS].
-- Only rewritten types end up below here.
----------------------------

-- NB: pattern match on True: we want only rewritten types sent to canEqLHS
-- This means we've rewritten any variables and reduced any type family redexes
-- See also Note [No top-level newtypes on RHS of representational equalities]
can_eq_nc' Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ps_ty1 Xi
ty2 Xi
ps_ty2
  | Just CanEqLHS
can_eq_lhs1 <- Xi -> Maybe CanEqLHS
canEqLHS_maybe Xi
ty1
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
NotSwapped CanEqLHS
can_eq_lhs1 Xi
ps_ty1 Xi
ty2 Xi
ps_ty2

  | Just CanEqLHS
can_eq_lhs2 <- Xi -> Maybe CanEqLHS
canEqLHS_maybe Xi
ty2
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
IsSwapped CanEqLHS
can_eq_lhs2 Xi
ps_ty2 Xi
ty1 Xi
ps_ty1

     -- If the type is TyConApp tc1 args1, then args1 really can't be less
     -- than tyConArity tc1. It could be *more* than tyConArity, but then we
     -- should have handled the case as an AppTy. That case only fires if
     -- _both_ sides of the equality are AppTy-like... but if one side is
     -- AppTy-like and the other isn't (and it also isn't a variable or
     -- saturated type family application, both of which are handled by
     -- can_eq_nc'), we're in a failure mode and can just fall through.

----------------------------
-- Fall-through. Give up.
----------------------------

-- We've rewritten and the types don't match. Give up.
can_eq_nc' Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel Xi
_ Xi
ps_ty1 Xi
_ Xi
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_nc' catch-all case" (Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ps_ty1 SDoc -> SDoc -> SDoc
$$ Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ps_ty2)
       ; case EqRel
eq_rel of -- See Note [Unsolved equalities]
            EqRel
ReprEq -> Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredStatus -> CtEvidence -> Ct
mkIrredCt CtIrredStatus
OtherCIS CtEvidence
ev)
            EqRel
NomEq  -> Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredStatus -> CtEvidence -> Ct
mkIrredCt CtIrredStatus
InsolubleCIS CtEvidence
ev) }
          -- No need to call canEqFailure/canEqHardFailure because they
          -- rewrite, and the types involved here are already rewritten

{- Note [Unsolved equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have an unsolved equality like
  (a b ~R# Int)
that is not necessarily insoluble!  Maybe 'a' will turn out to be a newtype.
So we want to make it a potentially-soluble Irred not an insoluble one.
Missing this point is what caused #15431

Note [ForAllTy and typechecker equality]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Should GHC type-check the following program (adapted from #15740)?

  {-# LANGUAGE PolyKinds, ... #-}
  data D a
  type family F :: forall k. k -> Type
  type instance F = D

Due to the way F is declared, any instance of F must have a right-hand side
whose kind is equal to `forall k. k -> Type`. The kind of D is
`forall {k}. k -> Type`, which is very close, but technically uses distinct
Core:

  -----------------------------------------------------------
  | Source Haskell    | Core                                |
  -----------------------------------------------------------
  | forall  k.  <...> | ForAllTy (Bndr k Specified) (<...>) |
  | forall {k}. <...> | ForAllTy (Bndr k Inferred)  (<...>) |
  -----------------------------------------------------------

We could deem these kinds to be unequal, but that would imply rejecting
programs like the one above. Whether a kind variable binder ends up being
specified or inferred can be somewhat subtle, however, especially for kinds
that aren't explicitly written out in the source code (like in D above).
For now, we decide to not make the specified/inferred status of an invisible
type variable binder affect GHC's notion of typechecker equality
(see Note [Typechecker equality vs definitional equality] in
GHC.Tc.Utils.TcType). That is, we have the following:

  --------------------------------------------------
  | Type 1            | Type 2            | Equal? |
  --------------------|-----------------------------
  | forall k. <...>   | forall k. <...>   | Yes    |
  |                   | forall {k}. <...> | Yes    |
  |                   | forall k -> <...> | No     |
  --------------------------------------------------
  | forall {k}. <...> | forall k. <...>   | Yes    |
  |                   | forall {k}. <...> | Yes    |
  |                   | forall k -> <...> | No     |
  --------------------------------------------------
  | forall k -> <...> | forall k. <...>   | No     |
  |                   | forall {k}. <...> | No     |
  |                   | forall k -> <...> | Yes    |
  --------------------------------------------------

We implement this nuance by using the GHC.Types.Var.sameVis function in
GHC.Tc.Solver.Canonical.canEqNC and GHC.Tc.Utils.TcType.tcEqType, which
respect typechecker equality. sameVis puts both forms of invisible type
variable binders into the same equivalence class.

Note that we do /not/ use sameVis in GHC.Core.Type.eqType, which implements
/definitional/ equality, a slighty more coarse-grained notion of equality
(see Note [Non-trivial definitional equality] in GHC.Core.TyCo.Rep) that does
not consider the ArgFlag of ForAllTys at all. That is, eqType would equate all
of forall k. <...>, forall {k}. <...>, and forall k -> <...>.
-}

---------------------------------
can_eq_nc_forall :: CtEvidence -> EqRel
                 -> Type -> Type    -- LHS and RHS
                 -> TcS (StopOrContinue Ct)
-- (forall as. phi1) ~ (forall bs. phi2)
-- Check for length match of as, bs
-- Then build an implication constraint: forall as. phi1 ~ phi2[as/bs]
-- But remember also to unify the kinds of as and bs
--  (this is the 'go' loop), and actually substitute phi2[as |> cos / bs]
-- Remember also that we might have forall z (a:z). blah
--  so we must proceed one binder at a time (#13879)

can_eq_nc_forall :: CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
can_eq_nc_forall CtEvidence
ev EqRel
eq_rel Xi
s1 Xi
s2
 | CtWanted { ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc, ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
orig_dest } <- CtEvidence
ev
 = do { let free_tvs :: VarSet
free_tvs       = [Xi] -> VarSet
tyCoVarsOfTypes [Xi
s1,Xi
s2]
            ([VarBndr TyVar ArgFlag]
bndrs1, Xi
phi1) = Xi -> ([VarBndr TyVar ArgFlag], Xi)
tcSplitForAllTyVarBinders Xi
s1
            ([VarBndr TyVar ArgFlag]
bndrs2, Xi
phi2) = Xi -> ([VarBndr TyVar ArgFlag], Xi)
tcSplitForAllTyVarBinders Xi
s2
      ; if Bool -> Bool
not ([VarBndr TyVar ArgFlag] -> [VarBndr TyVar ArgFlag] -> Bool
forall a b. [a] -> [b] -> Bool
equalLength [VarBndr TyVar ArgFlag]
bndrs1 [VarBndr TyVar ArgFlag]
bndrs2)
        then do { String -> SDoc -> TcS ()
traceTcS String
"Forall failure" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                     [SDoc] -> SDoc
vcat [ Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
s1, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
s2, [VarBndr TyVar ArgFlag] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [VarBndr TyVar ArgFlag]
bndrs1, [VarBndr TyVar ArgFlag] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [VarBndr TyVar ArgFlag]
bndrs2
                          , [ArgFlag] -> SDoc
forall a. Outputable a => a -> SDoc
ppr ((VarBndr TyVar ArgFlag -> ArgFlag)
-> [VarBndr TyVar ArgFlag] -> [ArgFlag]
forall a b. (a -> b) -> [a] -> [b]
map VarBndr TyVar ArgFlag -> ArgFlag
forall tv argf. VarBndr tv argf -> argf
binderArgFlag [VarBndr TyVar ArgFlag]
bndrs1)
                          , [ArgFlag] -> SDoc
forall a. Outputable a => a -> SDoc
ppr ((VarBndr TyVar ArgFlag -> ArgFlag)
-> [VarBndr TyVar ArgFlag] -> [ArgFlag]
forall a b. (a -> b) -> [a] -> [b]
map VarBndr TyVar ArgFlag -> ArgFlag
forall tv argf. VarBndr tv argf -> argf
binderArgFlag [VarBndr TyVar ArgFlag]
bndrs2) ]
                ; CtEvidence -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev Xi
s1 Xi
s2 }
        else
   do { String -> SDoc -> TcS ()
traceTcS String
"Creating implication for polytype equality" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
      ; let empty_subst1 :: TCvSubst
empty_subst1 = InScopeSet -> TCvSubst
mkEmptyTCvSubst (InScopeSet -> TCvSubst) -> InScopeSet -> TCvSubst
forall a b. (a -> b) -> a -> b
$ VarSet -> InScopeSet
mkInScopeSet VarSet
free_tvs
      ; (TCvSubst
subst1, [TyVar]
skol_tvs) <- TCvSubst -> [TyVar] -> TcS (TCvSubst, [TyVar])
tcInstSkolTyVarsX TCvSubst
empty_subst1 ([TyVar] -> TcS (TCvSubst, [TyVar]))
-> [TyVar] -> TcS (TCvSubst, [TyVar])
forall a b. (a -> b) -> a -> b
$
                              [VarBndr TyVar ArgFlag] -> [TyVar]
forall tv argf. [VarBndr tv argf] -> [tv]
binderVars [VarBndr TyVar ArgFlag]
bndrs1

      ; let skol_info :: SkolemInfo
skol_info = Xi -> SkolemInfo
UnifyForAllSkol Xi
phi1
            phi1' :: Xi
phi1' = HasCallStack => TCvSubst -> Xi -> Xi
TCvSubst -> Xi -> Xi
substTy TCvSubst
subst1 Xi
phi1

            -- Unify the kinds, extend the substitution
            go :: [TcTyVar] -> TCvSubst -> [TyVarBinder]
               -> TcS (TcCoercion, Cts)
            go :: [TyVar]
-> TCvSubst -> [VarBndr TyVar ArgFlag] -> TcS (TcCoercion, Bag Ct)
go (TyVar
skol_tv:[TyVar]
skol_tvs) TCvSubst
subst (VarBndr TyVar ArgFlag
bndr2:[VarBndr TyVar ArgFlag]
bndrs2)
              = do { let tv2 :: TyVar
tv2 = VarBndr TyVar ArgFlag -> TyVar
forall tv argf. VarBndr tv argf -> tv
binderVar VarBndr TyVar ArgFlag
bndr2
                   ; (TcCoercion
kind_co, Bag Ct
wanteds1) <- CtLoc -> Role -> Xi -> Xi -> TcS (TcCoercion, Bag Ct)
unify CtLoc
loc Role
Nominal (TyVar -> Xi
tyVarKind TyVar
skol_tv)
                                                  (HasCallStack => TCvSubst -> Xi -> Xi
TCvSubst -> Xi -> Xi
substTy TCvSubst
subst (TyVar -> Xi
tyVarKind TyVar
tv2))
                   ; let subst' :: TCvSubst
subst' = TCvSubst -> TyVar -> Xi -> TCvSubst
extendTvSubstAndInScope TCvSubst
subst TyVar
tv2
                                       (Xi -> TcCoercion -> Xi
mkCastTy (TyVar -> Xi
mkTyVarTy TyVar
skol_tv) TcCoercion
kind_co)
                         -- skol_tv is already in the in-scope set, but the
                         -- free vars of kind_co are not; hence "...AndInScope"
                   ; (TcCoercion
co, Bag Ct
wanteds2) <- [TyVar]
-> TCvSubst -> [VarBndr TyVar ArgFlag] -> TcS (TcCoercion, Bag Ct)
go [TyVar]
skol_tvs TCvSubst
subst' [VarBndr TyVar ArgFlag]
bndrs2
                   ; (TcCoercion, Bag Ct) -> TcS (TcCoercion, Bag Ct)
forall (m :: * -> *) a. Monad m => a -> m a
return ( TyVar -> TcCoercion -> TcCoercion -> TcCoercion
mkTcForAllCo TyVar
skol_tv TcCoercion
kind_co TcCoercion
co
                            , Bag Ct
wanteds1 Bag Ct -> Bag Ct -> Bag Ct
forall a. Bag a -> Bag a -> Bag a
`unionBags` Bag Ct
wanteds2 ) }

            -- Done: unify phi1 ~ phi2
            go [] TCvSubst
subst [VarBndr TyVar ArgFlag]
bndrs2
              = ASSERT( null bndrs2 )
                CtLoc -> Role -> Xi -> Xi -> TcS (TcCoercion, Bag Ct)
unify CtLoc
loc (EqRel -> Role
eqRelRole EqRel
eq_rel) Xi
phi1' (TCvSubst -> Xi -> Xi
substTyUnchecked TCvSubst
subst Xi
phi2)

            go [TyVar]
_ TCvSubst
_ [VarBndr TyVar ArgFlag]
_ = String -> TcS (TcCoercion, Bag Ct)
forall a. String -> a
panic String
"cna_eq_nc_forall"  -- case (s:ss) []

            empty_subst2 :: TCvSubst
empty_subst2 = InScopeSet -> TCvSubst
mkEmptyTCvSubst (TCvSubst -> InScopeSet
getTCvInScope TCvSubst
subst1)

      ; (TcLevel
lvl, (TcCoercion
all_co, Bag Ct
wanteds)) <- SDoc
-> TcS (TcCoercion, Bag Ct) -> TcS (TcLevel, (TcCoercion, Bag Ct))
forall a. SDoc -> TcS a -> TcS (TcLevel, a)
pushLevelNoWorkList (SkolemInfo -> SDoc
forall a. Outputable a => a -> SDoc
ppr SkolemInfo
skol_info) (TcS (TcCoercion, Bag Ct) -> TcS (TcLevel, (TcCoercion, Bag Ct)))
-> TcS (TcCoercion, Bag Ct) -> TcS (TcLevel, (TcCoercion, Bag Ct))
forall a b. (a -> b) -> a -> b
$
                                    [TyVar]
-> TCvSubst -> [VarBndr TyVar ArgFlag] -> TcS (TcCoercion, Bag Ct)
go [TyVar]
skol_tvs TCvSubst
empty_subst2 [VarBndr TyVar ArgFlag]
bndrs2
      ; TcLevel -> SkolemInfo -> [TyVar] -> Bag Ct -> TcS ()
emitTvImplicationTcS TcLevel
lvl SkolemInfo
skol_info [TyVar]
skol_tvs Bag Ct
wanteds

      ; TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
orig_dest TcCoercion
all_co
      ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Deferred polytype equality" } }

 | Bool
otherwise
 = do { String -> SDoc -> TcS ()
traceTcS String
"Omitting decomposition of given polytype equality" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
        Xi -> Xi -> SDoc
pprEq Xi
s1 Xi
s2    -- See Note [Do not decompose given polytype equalities]
      ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Discard given polytype equality" }

 where
    unify :: CtLoc -> Role -> TcType -> TcType -> TcS (TcCoercion, Cts)
    -- This version returns the wanted constraint rather
    -- than putting it in the work list
    unify :: CtLoc -> Role -> Xi -> Xi -> TcS (TcCoercion, Bag Ct)
unify CtLoc
loc Role
role Xi
ty1 Xi
ty2
      | Xi
ty1 HasDebugCallStack => Xi -> Xi -> Bool
Xi -> Xi -> Bool
`tcEqType` Xi
ty2
      = (TcCoercion, Bag Ct) -> TcS (TcCoercion, Bag Ct)
forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Xi -> TcCoercion
mkTcReflCo Role
role Xi
ty1, Bag Ct
forall a. Bag a
emptyBag)
      | Bool
otherwise
      = do { (CtEvidence
wanted, TcCoercion
co) <- CtLoc -> Role -> Xi -> Xi -> TcS (CtEvidence, TcCoercion)
newWantedEq CtLoc
loc Role
role Xi
ty1 Xi
ty2
           ; (TcCoercion, Bag Ct) -> TcS (TcCoercion, Bag Ct)
forall (m :: * -> *) a. Monad m => a -> m a
return (TcCoercion
co, Ct -> Bag Ct
forall a. a -> Bag a
unitBag (CtEvidence -> Ct
mkNonCanonical CtEvidence
wanted)) }

---------------------------------
-- | Compare types for equality, while zonking as necessary. Gives up
-- as soon as it finds that two types are not equal.
-- This is quite handy when some unification has made two
-- types in an inert Wanted to be equal. We can discover the equality without
-- rewriting, which is sometimes very expensive (in the case of type functions).
-- In particular, this function makes a ~20% improvement in test case
-- perf/compiler/T5030.
--
-- Returns either the (partially zonked) types in the case of
-- inequality, or the one type in the case of equality. canEqReflexive is
-- a good next step in the 'Right' case. Returning 'Left' is always safe.
--
-- NB: This does *not* look through type synonyms. In fact, it treats type
-- synonyms as rigid constructors. In the future, it might be convenient
-- to look at only those arguments of type synonyms that actually appear
-- in the synonym RHS. But we're not there yet.
zonk_eq_types :: TcType -> TcType -> TcS (Either (Pair TcType) TcType)
zonk_eq_types :: Xi -> Xi -> TcS (Either (Pair Xi) Xi)
zonk_eq_types = Xi -> Xi -> TcS (Either (Pair Xi) Xi)
go
  where
    go :: Xi -> Xi -> TcS (Either (Pair Xi) Xi)
go (TyVarTy TyVar
tv1) (TyVarTy TyVar
tv2) = TyVar -> TyVar -> TcS (Either (Pair Xi) Xi)
tyvar_tyvar TyVar
tv1 TyVar
tv2
    go (TyVarTy TyVar
tv1) Xi
ty2           = SwapFlag -> TyVar -> Xi -> TcS (Either (Pair Xi) Xi)
tyvar SwapFlag
NotSwapped TyVar
tv1 Xi
ty2
    go Xi
ty1 (TyVarTy TyVar
tv2)           = SwapFlag -> TyVar -> Xi -> TcS (Either (Pair Xi) Xi)
tyvar SwapFlag
IsSwapped  TyVar
tv2 Xi
ty1

    -- We handle FunTys explicitly here despite the fact that they could also be
    -- treated as an application. Why? Well, for one it's cheaper to just look
    -- at two types (the argument and result types) than four (the argument,
    -- result, and their RuntimeReps). Also, we haven't completely zonked yet,
    -- so we may run into an unzonked type variable while trying to compute the
    -- RuntimeReps of the argument and result types. This can be observed in
    -- testcase tc269.
    go Xi
ty1 Xi
ty2
      | Just (Scaled Xi
w1 Xi
arg1, Xi
res1) <- Maybe (Scaled Xi, Xi)
split1
      , Just (Scaled Xi
w2 Xi
arg2, Xi
res2) <- Maybe (Scaled Xi, Xi)
split2
      , Xi -> Xi -> Bool
eqType Xi
w1 Xi
w2
      = do { Either (Pair Xi) Xi
res_a <- Xi -> Xi -> TcS (Either (Pair Xi) Xi)
go Xi
arg1 Xi
arg2
           ; Either (Pair Xi) Xi
res_b <- Xi -> Xi -> TcS (Either (Pair Xi) Xi)
go Xi
res1 Xi
res2
           ; Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a. Monad m => a -> m a
return (Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi))
-> Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall a b. (a -> b) -> a -> b
$ (Xi -> Xi -> Xi)
-> Either (Pair Xi) Xi
-> Either (Pair Xi) Xi
-> Either (Pair Xi) Xi
forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev (Xi -> Xi -> Xi -> Xi
mkVisFunTy Xi
w1) Either (Pair Xi) Xi
res_b Either (Pair Xi) Xi
res_a
           }
      | Maybe (Scaled Xi, Xi) -> Bool
forall a. Maybe a -> Bool
isJust Maybe (Scaled Xi, Xi)
split1 Bool -> Bool -> Bool
|| Maybe (Scaled Xi, Xi) -> Bool
forall a. Maybe a -> Bool
isJust Maybe (Scaled Xi, Xi)
split2
      = Xi -> Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a b.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out Xi
ty1 Xi
ty2
      where
        split1 :: Maybe (Scaled Xi, Xi)
split1 = Xi -> Maybe (Scaled Xi, Xi)
tcSplitFunTy_maybe Xi
ty1
        split2 :: Maybe (Scaled Xi, Xi)
split2 = Xi -> Maybe (Scaled Xi, Xi)
tcSplitFunTy_maybe Xi
ty2

    go Xi
ty1 Xi
ty2
      | Just (TyCon
tc1, [Xi]
tys1) <- HasDebugCallStack => Xi -> Maybe (TyCon, [Xi])
Xi -> Maybe (TyCon, [Xi])
repSplitTyConApp_maybe Xi
ty1
      , Just (TyCon
tc2, [Xi]
tys2) <- HasDebugCallStack => Xi -> Maybe (TyCon, [Xi])
Xi -> Maybe (TyCon, [Xi])
repSplitTyConApp_maybe Xi
ty2
      = if TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2 Bool -> Bool -> Bool
&& [Xi]
tys1 [Xi] -> [Xi] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [Xi]
tys2
          -- Crucial to check for equal-length args, because
          -- we cannot assume that the two args to 'go' have
          -- the same kind.  E.g go (Proxy *      (Maybe Int))
          --                        (Proxy (*->*) Maybe)
          -- We'll call (go (Maybe Int) Maybe)
          -- See #13083
        then TyCon -> [Xi] -> [Xi] -> TcS (Either (Pair Xi) Xi)
tycon TyCon
tc1 [Xi]
tys1 [Xi]
tys2
        else Xi -> Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a b.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out Xi
ty1 Xi
ty2

    go Xi
ty1 Xi
ty2
      | Just (Xi
ty1a, Xi
ty1b) <- Xi -> Maybe (Xi, Xi)
tcRepSplitAppTy_maybe Xi
ty1
      , Just (Xi
ty2a, Xi
ty2b) <- Xi -> Maybe (Xi, Xi)
tcRepSplitAppTy_maybe Xi
ty2
      = do { Either (Pair Xi) Xi
res_a <- Xi -> Xi -> TcS (Either (Pair Xi) Xi)
go Xi
ty1a Xi
ty2a
           ; Either (Pair Xi) Xi
res_b <- Xi -> Xi -> TcS (Either (Pair Xi) Xi)
go Xi
ty1b Xi
ty2b
           ; Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a. Monad m => a -> m a
return (Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi))
-> Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall a b. (a -> b) -> a -> b
$ (Xi -> Xi -> Xi)
-> Either (Pair Xi) Xi
-> Either (Pair Xi) Xi
-> Either (Pair Xi) Xi
forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev Xi -> Xi -> Xi
mkAppTy Either (Pair Xi) Xi
res_b Either (Pair Xi) Xi
res_a }

    go ty1 :: Xi
ty1@(LitTy TyLit
lit1) (LitTy TyLit
lit2)
      | TyLit
lit1 TyLit -> TyLit -> Bool
forall a. Eq a => a -> a -> Bool
== TyLit
lit2
      = Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a. Monad m => a -> m a
return (Xi -> Either (Pair Xi) Xi
forall a b. b -> Either a b
Right Xi
ty1)

    go Xi
ty1 Xi
ty2 = Xi -> Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a b.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out Xi
ty1 Xi
ty2
      -- We don't handle more complex forms here

    bale_out :: a -> a -> m (Either (Pair a) b)
bale_out a
ty1 a
ty2 = Either (Pair a) b -> m (Either (Pair a) b)
forall (m :: * -> *) a. Monad m => a -> m a
return (Either (Pair a) b -> m (Either (Pair a) b))
-> Either (Pair a) b -> m (Either (Pair a) b)
forall a b. (a -> b) -> a -> b
$ Pair a -> Either (Pair a) b
forall a b. a -> Either a b
Left (a -> a -> Pair a
forall a. a -> a -> Pair a
Pair a
ty1 a
ty2)

    tyvar :: SwapFlag -> TcTyVar -> TcType
          -> TcS (Either (Pair TcType) TcType)
      -- Try to do as little as possible, as anything we do here is redundant
      -- with rewriting. In particular, no need to zonk kinds. That's why
      -- we don't use the already-defined zonking functions
    tyvar :: SwapFlag -> TyVar -> Xi -> TcS (Either (Pair Xi) Xi)
tyvar SwapFlag
swapped TyVar
tv Xi
ty
      = case TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv of
          MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
            -> do { MetaDetails
cts <- IORef MetaDetails -> TcS MetaDetails
forall a. TcRef a -> TcS a
readTcRef IORef MetaDetails
ref
                  ; case MetaDetails
cts of
                      MetaDetails
Flexi        -> TcS (Either (Pair Xi) Xi)
give_up
                      Indirect Xi
ty' -> do { TyVar -> Xi -> TcS ()
forall a a. (Outputable a, Outputable a) => a -> a -> TcS ()
trace_indirect TyVar
tv Xi
ty'
                                         ; SwapFlag
-> (Xi -> Xi -> TcS (Either (Pair Xi) Xi))
-> Xi
-> Xi
-> TcS (Either (Pair Xi) Xi)
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped Xi -> Xi -> TcS (Either (Pair Xi) Xi)
go Xi
ty' Xi
ty } }
          TcTyVarDetails
_ -> TcS (Either (Pair Xi) Xi)
give_up
      where
        give_up :: TcS (Either (Pair Xi) Xi)
give_up = Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a. Monad m => a -> m a
return (Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi))
-> Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall a b. (a -> b) -> a -> b
$ Pair Xi -> Either (Pair Xi) Xi
forall a b. a -> Either a b
Left (Pair Xi -> Either (Pair Xi) Xi) -> Pair Xi -> Either (Pair Xi) Xi
forall a b. (a -> b) -> a -> b
$ SwapFlag -> (Xi -> Xi -> Pair Xi) -> Xi -> Xi -> Pair Xi
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped Xi -> Xi -> Pair Xi
forall a. a -> a -> Pair a
Pair (TyVar -> Xi
mkTyVarTy TyVar
tv) Xi
ty

    tyvar_tyvar :: TyVar -> TyVar -> TcS (Either (Pair Xi) Xi)
tyvar_tyvar TyVar
tv1 TyVar
tv2
      | TyVar
tv1 TyVar -> TyVar -> Bool
forall a. Eq a => a -> a -> Bool
== TyVar
tv2 = Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a. Monad m => a -> m a
return (Xi -> Either (Pair Xi) Xi
forall a b. b -> Either a b
Right (TyVar -> Xi
mkTyVarTy TyVar
tv1))
      | Bool
otherwise  = do { (Xi
ty1', Bool
progress1) <- TyVar -> TcS (Xi, Bool)
quick_zonk TyVar
tv1
                        ; (Xi
ty2', Bool
progress2) <- TyVar -> TcS (Xi, Bool)
quick_zonk TyVar
tv2
                        ; if Bool
progress1 Bool -> Bool -> Bool
|| Bool
progress2
                          then Xi -> Xi -> TcS (Either (Pair Xi) Xi)
go Xi
ty1' Xi
ty2'
                          else Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a. Monad m => a -> m a
return (Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi))
-> Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall a b. (a -> b) -> a -> b
$ Pair Xi -> Either (Pair Xi) Xi
forall a b. a -> Either a b
Left (Xi -> Xi -> Pair Xi
forall a. a -> a -> Pair a
Pair (TyVar -> Xi
TyVarTy TyVar
tv1) (TyVar -> Xi
TyVarTy TyVar
tv2)) }

    trace_indirect :: a -> a -> TcS ()
trace_indirect a
tv a
ty
       = String -> SDoc -> TcS ()
traceTcS String
"Following filled tyvar (zonk_eq_types)"
                  (a -> SDoc
forall a. Outputable a => a -> SDoc
ppr a
tv SDoc -> SDoc -> SDoc
<+> SDoc
equals SDoc -> SDoc -> SDoc
<+> a -> SDoc
forall a. Outputable a => a -> SDoc
ppr a
ty)

    quick_zonk :: TyVar -> TcS (Xi, Bool)
quick_zonk TyVar
tv = case TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv of
      MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
        -> do { MetaDetails
cts <- IORef MetaDetails -> TcS MetaDetails
forall a. TcRef a -> TcS a
readTcRef IORef MetaDetails
ref
              ; case MetaDetails
cts of
                  MetaDetails
Flexi        -> (Xi, Bool) -> TcS (Xi, Bool)
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Xi
TyVarTy TyVar
tv, Bool
False)
                  Indirect Xi
ty' -> do { TyVar -> Xi -> TcS ()
forall a a. (Outputable a, Outputable a) => a -> a -> TcS ()
trace_indirect TyVar
tv Xi
ty'
                                     ; (Xi, Bool) -> TcS (Xi, Bool)
forall (m :: * -> *) a. Monad m => a -> m a
return (Xi
ty', Bool
True) } }
      TcTyVarDetails
_ -> (Xi, Bool) -> TcS (Xi, Bool)
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Xi
TyVarTy TyVar
tv, Bool
False)

      -- This happens for type families, too. But recall that failure
      -- here just means to try harder, so it's OK if the type function
      -- isn't injective.
    tycon :: TyCon -> [TcType] -> [TcType]
          -> TcS (Either (Pair TcType) TcType)
    tycon :: TyCon -> [Xi] -> [Xi] -> TcS (Either (Pair Xi) Xi)
tycon TyCon
tc [Xi]
tys1 [Xi]
tys2
      = do { [Either (Pair Xi) Xi]
results <- (Xi -> Xi -> TcS (Either (Pair Xi) Xi))
-> [Xi] -> [Xi] -> TcS [Either (Pair Xi) Xi]
forall (m :: * -> *) a b c.
Applicative m =>
(a -> b -> m c) -> [a] -> [b] -> m [c]
zipWithM Xi -> Xi -> TcS (Either (Pair Xi) Xi)
go [Xi]
tys1 [Xi]
tys2
           ; Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall (m :: * -> *) a. Monad m => a -> m a
return (Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi))
-> Either (Pair Xi) Xi -> TcS (Either (Pair Xi) Xi)
forall a b. (a -> b) -> a -> b
$ case [Either (Pair Xi) Xi] -> Either (Pair [Xi]) [Xi]
combine_results [Either (Pair Xi) Xi]
results of
               Left Pair [Xi]
tys  -> Pair Xi -> Either (Pair Xi) Xi
forall a b. a -> Either a b
Left (TyCon -> [Xi] -> Xi
mkTyConApp TyCon
tc ([Xi] -> Xi) -> Pair [Xi] -> Pair Xi
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair [Xi]
tys)
               Right [Xi]
tys -> Xi -> Either (Pair Xi) Xi
forall a b. b -> Either a b
Right (TyCon -> [Xi] -> Xi
mkTyConApp TyCon
tc [Xi]
tys) }

    combine_results :: [Either (Pair TcType) TcType]
                    -> Either (Pair [TcType]) [TcType]
    combine_results :: [Either (Pair Xi) Xi] -> Either (Pair [Xi]) [Xi]
combine_results = (Pair [Xi] -> Pair [Xi])
-> ([Xi] -> [Xi])
-> Either (Pair [Xi]) [Xi]
-> Either (Pair [Xi]) [Xi]
forall (p :: * -> * -> *) a b c d.
Bifunctor p =>
(a -> b) -> (c -> d) -> p a c -> p b d
bimap (([Xi] -> [Xi]) -> Pair [Xi] -> Pair [Xi]
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap [Xi] -> [Xi]
forall a. [a] -> [a]
reverse) [Xi] -> [Xi]
forall a. [a] -> [a]
reverse (Either (Pair [Xi]) [Xi] -> Either (Pair [Xi]) [Xi])
-> ([Either (Pair Xi) Xi] -> Either (Pair [Xi]) [Xi])
-> [Either (Pair Xi) Xi]
-> Either (Pair [Xi]) [Xi]
forall b c a. (b -> c) -> (a -> b) -> a -> c
.
                      (Either (Pair [Xi]) [Xi]
 -> Either (Pair Xi) Xi -> Either (Pair [Xi]) [Xi])
-> Either (Pair [Xi]) [Xi]
-> [Either (Pair Xi) Xi]
-> Either (Pair [Xi]) [Xi]
forall (t :: * -> *) b a.
Foldable t =>
(b -> a -> b) -> b -> t a -> b
foldl' ((Xi -> [Xi] -> [Xi])
-> Either (Pair [Xi]) [Xi]
-> Either (Pair Xi) Xi
-> Either (Pair [Xi]) [Xi]
forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev (:)) ([Xi] -> Either (Pair [Xi]) [Xi]
forall a b. b -> Either a b
Right [])

      -- combine (in reverse) a new result onto an already-combined result
    combine_rev :: (a -> b -> c)
                -> Either (Pair b) b
                -> Either (Pair a) a
                -> Either (Pair c) c
    combine_rev :: (a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev a -> b -> c
f (Left Pair b
list) (Left Pair a
elt) = Pair c -> Either (Pair c) c
forall a b. a -> Either a b
Left (a -> b -> c
f (a -> b -> c) -> Pair a -> Pair (b -> c)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair a
elt     Pair (b -> c) -> Pair b -> Pair c
forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> Pair b
list)
    combine_rev a -> b -> c
f (Left Pair b
list) (Right a
ty) = Pair c -> Either (Pair c) c
forall a b. a -> Either a b
Left (a -> b -> c
f (a -> b -> c) -> Pair a -> Pair (b -> c)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> a -> Pair a
forall (f :: * -> *) a. Applicative f => a -> f a
pure a
ty Pair (b -> c) -> Pair b -> Pair c
forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> Pair b
list)
    combine_rev a -> b -> c
f (Right b
tys) (Left Pair a
elt) = Pair c -> Either (Pair c) c
forall a b. a -> Either a b
Left (a -> b -> c
f (a -> b -> c) -> Pair a -> Pair (b -> c)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair a
elt     Pair (b -> c) -> Pair b -> Pair c
forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> b -> Pair b
forall (f :: * -> *) a. Applicative f => a -> f a
pure b
tys)
    combine_rev a -> b -> c
f (Right b
tys) (Right a
ty) = c -> Either (Pair c) c
forall a b. b -> Either a b
Right (a -> b -> c
f a
ty b
tys)

{- See Note [Unwrap newtypes first]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
  newtype N m a = MkN (m a)
Then N will get a conservative, Nominal role for its second parameter 'a',
because it appears as an argument to the unknown 'm'. Now consider
  [W] N Maybe a  ~R#  N Maybe b

If we decompose, we'll get
  [W] a ~N# b

But if instead we unwrap we'll get
  [W] Maybe a ~R# Maybe b
which in turn gives us
  [W] a ~R# b
which is easier to satisfy.

Bottom line: unwrap newtypes before decomposing them!
c.f. #9123 comment:52,53 for a compelling example.

Note [Newtypes can blow the stack]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)
  newtype Y = MkY (Int -> Y)

and now wish to prove

  [W] X ~R Y

This Wanted will loop, expanding out the newtypes ever deeper looking
for a solid match or a solid discrepancy. Indeed, there is something
appropriate to this looping, because X and Y *do* have the same representation,
in the limit -- they're both (Fix ((->) Int)). However, no finitely-sized
coercion will ever witness it. This loop won't actually cause GHC to hang,
though, because we check our depth when unwrapping newtypes.

Note [Eager reflexivity check]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)

and

  [W] X ~R X

Naively, we would start unwrapping X and end up in a loop. Instead,
we do this eager reflexivity check. This is necessary only for representational
equality because the rewriter technology deals with the similar case
(recursive type families) for nominal equality.

Note that this check does not catch all cases, but it will catch the cases
we're most worried about, types like X above that are actually inhabited.

Here's another place where this reflexivity check is key:
Consider trying to prove (f a) ~R (f a). The AppTys in there can't
be decomposed, because representational equality isn't congruent with respect
to AppTy. So, when canonicalising the equality above, we get stuck and
would normally produce a CIrredCan. However, we really do want to
be able to solve (f a) ~R (f a). So, in the representational case only,
we do a reflexivity check.

(This would be sound in the nominal case, but unnecessary, and I [Richard
E.] am worried that it would slow down the common case.)
-}

------------------------
-- | We're able to unwrap a newtype. Update the bits accordingly.
can_eq_newtype_nc :: CtEvidence           -- ^ :: ty1 ~ ty2
                  -> SwapFlag
                  -> TcType                                    -- ^ ty1
                  -> ((Bag GlobalRdrElt, TcCoercion), TcType)  -- ^ :: ty1 ~ ty1'
                  -> TcType               -- ^ ty2
                  -> TcType               -- ^ ty2, with type synonyms
                  -> TcS (StopOrContinue Ct)
can_eq_newtype_nc :: CtEvidence
-> SwapFlag
-> Xi
-> ((Bag GlobalRdrElt, TcCoercion), Xi)
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_newtype_nc CtEvidence
ev SwapFlag
swapped Xi
ty1 ((Bag GlobalRdrElt
gres, TcCoercion
co), Xi
ty1') Xi
ty2 Xi
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_newtype_nc" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev, SwapFlag -> SDoc
forall a. Outputable a => a -> SDoc
ppr SwapFlag
swapped, TcCoercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcCoercion
co, Bag GlobalRdrElt -> SDoc
forall a. Outputable a => a -> SDoc
ppr Bag GlobalRdrElt
gres, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty1', Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty2 ]

         -- check for blowing our stack:
         -- See Note [Newtypes can blow the stack]
       ; CtLoc -> Xi -> TcS ()
checkReductionDepth (CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev) Xi
ty1

         -- Next, we record uses of newtype constructors, since coercing
         -- through newtypes is tantamount to using their constructors.
       ; [GlobalRdrElt] -> TcS ()
addUsedGREs [GlobalRdrElt]
gre_list
         -- If a newtype constructor was imported, don't warn about not
         -- importing it...
       ; (Name -> TcS ()) -> [Name] -> TcS ()
forall (t :: * -> *) (f :: * -> *) a b.
(Foldable t, Applicative f) =>
(a -> f b) -> t a -> f ()
traverse_ Name -> TcS ()
keepAlive ([Name] -> TcS ()) -> [Name] -> TcS ()
forall a b. (a -> b) -> a -> b
$ (GlobalRdrElt -> Name) -> [GlobalRdrElt] -> [Name]
forall a b. (a -> b) -> [a] -> [b]
map GlobalRdrElt -> Name
greMangledName [GlobalRdrElt]
gre_list
         -- ...and similarly, if a newtype constructor was defined in the same
         -- module, don't warn about it being unused.
         -- See Note [Tracking unused binding and imports] in GHC.Tc.Utils.

       ; CtEvidence
new_ev <- CtEvidence
-> SwapFlag
-> Xi
-> Xi
-> TcCoercion
-> TcCoercion
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped Xi
ty1' Xi
ps_ty2
                                     (TcCoercion -> TcCoercion
mkTcSymCo TcCoercion
co) (Role -> Xi -> TcCoercion
mkTcReflCo Role
Representational Xi
ps_ty2)
       ; Bool
-> CtEvidence
-> EqRel
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_nc Bool
False CtEvidence
new_ev EqRel
ReprEq Xi
ty1' Xi
ty1' Xi
ty2 Xi
ps_ty2 }
  where
    gre_list :: [GlobalRdrElt]
gre_list = Bag GlobalRdrElt -> [GlobalRdrElt]
forall a. Bag a -> [a]
bagToList Bag GlobalRdrElt
gres

---------
-- ^ Decompose a type application.
-- All input types must be rewritten. See Note [Canonicalising type applications]
-- Nominal equality only!
can_eq_app :: CtEvidence       -- :: s1 t1 ~N s2 t2
           -> Xi -> Xi         -- s1 t1
           -> Xi -> Xi         -- s2 t2
           -> TcS (StopOrContinue Ct)

-- AppTys only decompose for nominal equality, so this case just leads
-- to an irreducible constraint; see typecheck/should_compile/T10494
-- See Note [Decomposing AppTy at representational role]
can_eq_app :: CtEvidence -> Xi -> Xi -> Xi -> Xi -> TcS (StopOrContinue Ct)
can_eq_app CtEvidence
ev Xi
s1 Xi
t1 Xi
s2 Xi
t2
  | CtDerived {} <- CtEvidence
ev
  = do { CtLoc -> [Role] -> [Xi] -> [Xi] -> TcS ()
unifyDeriveds CtLoc
loc [Role
Nominal, Role
Nominal] [Xi
s1, Xi
t1] [Xi
s2, Xi
t2]
       ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Decomposed [D] AppTy" }

  | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest } <- CtEvidence
ev
  = do { TcCoercion
co_s <- CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted CtLoc
loc Role
Nominal Xi
s1 Xi
s2
       ; let arg_loc :: CtLoc
arg_loc
               | Xi -> Bool
isNextArgVisible Xi
s1 = CtLoc
loc
               | Bool
otherwise           = CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
updateCtLocOrigin CtLoc
loc CtOrigin -> CtOrigin
toInvisibleOrigin
       ; TcCoercion
co_t <- CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted CtLoc
arg_loc Role
Nominal Xi
t1 Xi
t2
       ; let co :: TcCoercion
co = TcCoercion -> TcCoercion -> TcCoercion
mkAppCo TcCoercion
co_s TcCoercion
co_t
       ; TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest TcCoercion
co
       ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Decomposed [W] AppTy" }

    -- If there is a ForAll/(->) mismatch, the use of the Left coercion
    -- below is ill-typed, potentially leading to a panic in splitTyConApp
    -- Test case: typecheck/should_run/Typeable1
    -- We could also include this mismatch check above (for W and D), but it's slow
    -- and we'll get a better error message not doing it
  | Xi
s1k Xi -> Xi -> Bool
`mismatches` Xi
s2k
  = CtEvidence -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev (Xi
s1 Xi -> Xi -> Xi
`mkAppTy` Xi
t1) (Xi
s2 Xi -> Xi -> Xi
`mkAppTy` Xi
t2)

  | CtGiven { ctev_evar :: CtEvidence -> TyVar
ctev_evar = TyVar
evar } <- CtEvidence
ev
  = do { let co :: TcCoercion
co   = TyVar -> TcCoercion
mkTcCoVarCo TyVar
evar
             co_s :: TcCoercion
co_s = LeftOrRight -> TcCoercion -> TcCoercion
mkTcLRCo LeftOrRight
CLeft  TcCoercion
co
             co_t :: TcCoercion
co_t = LeftOrRight -> TcCoercion -> TcCoercion
mkTcLRCo LeftOrRight
CRight TcCoercion
co
       ; CtEvidence
evar_s <- CtLoc -> (Xi, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc ( CtEvidence -> Xi -> Xi -> Xi
mkTcEqPredLikeEv CtEvidence
ev Xi
s1 Xi
s2
                                     , TcCoercion -> EvTerm
evCoercion TcCoercion
co_s )
       ; CtEvidence
evar_t <- CtLoc -> (Xi, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc ( CtEvidence -> Xi -> Xi -> Xi
mkTcEqPredLikeEv CtEvidence
ev Xi
t1 Xi
t2
                                     , TcCoercion -> EvTerm
evCoercion TcCoercion
co_t )
       ; [CtEvidence] -> TcS ()
emitWorkNC [CtEvidence
evar_t]
       ; CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqNC CtEvidence
evar_s EqRel
NomEq Xi
s1 Xi
s2 }

  where
    loc :: CtLoc
loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev

    s1k :: Xi
s1k = HasDebugCallStack => Xi -> Xi
Xi -> Xi
tcTypeKind Xi
s1
    s2k :: Xi
s2k = HasDebugCallStack => Xi -> Xi
Xi -> Xi
tcTypeKind Xi
s2

    Xi
k1 mismatches :: Xi -> Xi -> Bool
`mismatches` Xi
k2
      =  Xi -> Bool
isForAllTy Xi
k1 Bool -> Bool -> Bool
&& Bool -> Bool
not (Xi -> Bool
isForAllTy Xi
k2)
      Bool -> Bool -> Bool
|| Bool -> Bool
not (Xi -> Bool
isForAllTy Xi
k1) Bool -> Bool -> Bool
&& Xi -> Bool
isForAllTy Xi
k2

-----------------------
-- | Break apart an equality over a casted type
-- looking like   (ty1 |> co1) ~ ty2   (modulo a swap-flag)
canEqCast :: Bool         -- are both types rewritten?
          -> CtEvidence
          -> EqRel
          -> SwapFlag
          -> TcType -> Coercion   -- LHS (res. RHS), ty1 |> co1
          -> TcType -> TcType     -- RHS (res. LHS), ty2 both normal and pretty
          -> TcS (StopOrContinue Ct)
canEqCast :: Bool
-> CtEvidence
-> EqRel
-> SwapFlag
-> Xi
-> TcCoercion
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCast Bool
rewritten CtEvidence
ev EqRel
eq_rel SwapFlag
swapped Xi
ty1 TcCoercion
co1 Xi
ty2 Xi
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"Decomposing cast" ([SDoc] -> SDoc
vcat [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
                                           , Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty1 SDoc -> SDoc -> SDoc
<+> String -> SDoc
text String
"|>" SDoc -> SDoc -> SDoc
<+> TcCoercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcCoercion
co1
                                           , Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ps_ty2 ])
       ; CtEvidence
new_ev <- CtEvidence
-> SwapFlag
-> Xi
-> Xi
-> TcCoercion
-> TcCoercion
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped Xi
ty1 Xi
ps_ty2
                                     (Role -> Xi -> TcCoercion -> TcCoercion
mkTcGReflRightCo Role
role Xi
ty1 TcCoercion
co1)
                                     (Role -> Xi -> TcCoercion
mkTcReflCo Role
role Xi
ps_ty2)
       ; Bool
-> CtEvidence
-> EqRel
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
can_eq_nc Bool
rewritten CtEvidence
new_ev EqRel
eq_rel Xi
ty1 Xi
ty1 Xi
ty2 Xi
ps_ty2 }
  where
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

------------------------
canTyConApp :: CtEvidence -> EqRel
            -> TyCon -> [TcType]
            -> TyCon -> [TcType]
            -> TcS (StopOrContinue Ct)
-- See Note [Decomposing TyConApps]
-- Neither tc1 nor tc2 is a saturated funTyCon
canTyConApp :: CtEvidence
-> EqRel
-> TyCon
-> [Xi]
-> TyCon
-> [Xi]
-> TcS (StopOrContinue Ct)
canTyConApp CtEvidence
ev EqRel
eq_rel TyCon
tc1 [Xi]
tys1 TyCon
tc2 [Xi]
tys2
  | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2
  , [Xi]
tys1 [Xi] -> [Xi] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [Xi]
tys2
  = do { InertSet
inerts <- TcS InertSet
getTcSInerts
       ; if InertSet -> Bool
can_decompose InertSet
inerts
         then CtEvidence
-> EqRel -> TyCon -> [Xi] -> [Xi] -> TcS (StopOrContinue Ct)
canDecomposableTyConAppOK CtEvidence
ev EqRel
eq_rel TyCon
tc1 [Xi]
tys1 [Xi]
tys2
         else CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqFailure CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ty2 }

  -- See Note [Skolem abstract data] (at tyConSkolem)
  | TyCon -> Bool
tyConSkolem TyCon
tc1 Bool -> Bool -> Bool
|| TyCon -> Bool
tyConSkolem TyCon
tc2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canTyConApp: skolem abstract" (TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
tc1 SDoc -> SDoc -> SDoc
$$ TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
tc2)
       ; Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredStatus -> CtEvidence -> Ct
mkIrredCt CtIrredStatus
OtherCIS CtEvidence
ev) }

  -- Fail straight away for better error messages
  -- See Note [Use canEqFailure in canDecomposableTyConApp]
  | EqRel
eq_rel EqRel -> EqRel -> Bool
forall a. Eq a => a -> a -> Bool
== EqRel
ReprEq Bool -> Bool -> Bool
&& Bool -> Bool
not (TyCon -> Role -> Bool
isGenerativeTyCon TyCon
tc1 Role
Representational Bool -> Bool -> Bool
&&
                             TyCon -> Role -> Bool
isGenerativeTyCon TyCon
tc2 Role
Representational)
  = CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqFailure CtEvidence
ev EqRel
eq_rel Xi
ty1 Xi
ty2
  | Bool
otherwise
  = CtEvidence -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev Xi
ty1 Xi
ty2
  where
    -- Reconstruct the types for error messages. This would do
    -- the wrong thing (from a pretty printing point of view)
    -- for functions, because we've lost the AnonArgFlag; but
    -- in fact we never call canTyConApp on a saturated FunTyCon
    ty1 :: Xi
ty1 = TyCon -> [Xi] -> Xi
mkTyConApp TyCon
tc1 [Xi]
tys1
    ty2 :: Xi
ty2 = TyCon -> [Xi] -> Xi
mkTyConApp TyCon
tc2 [Xi]
tys2

    loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    pred :: Xi
pred = CtEvidence -> Xi
ctEvPred CtEvidence
ev

     -- See Note [Decomposing equality]
    can_decompose :: InertSet -> Bool
can_decompose InertSet
inerts
      =  TyCon -> Role -> Bool
isInjectiveTyCon TyCon
tc1 (EqRel -> Role
eqRelRole EqRel
eq_rel)
      Bool -> Bool -> Bool
|| (CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev CtFlavour -> CtFlavour -> Bool
forall a. Eq a => a -> a -> Bool
/= CtFlavour
Given Bool -> Bool -> Bool
&& Bag Ct -> Bool
forall a. Bag a -> Bool
isEmptyBag (CtLoc -> Xi -> InertSet -> Bag Ct
matchableGivens CtLoc
loc Xi
pred InertSet
inerts))

{-
Note [Use canEqFailure in canDecomposableTyConApp]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We must use canEqFailure, not canEqHardFailure here, because there is
the possibility of success if working with a representational equality.
Here is one case:

  type family TF a where TF Char = Bool
  data family DF a
  newtype instance DF Bool = MkDF Int

Suppose we are canonicalising (Int ~R DF (TF a)), where we don't yet
know `a`. This is *not* a hard failure, because we might soon learn
that `a` is, in fact, Char, and then the equality succeeds.

Here is another case:

  [G] Age ~R Int

where Age's constructor is not in scope. We don't want to report
an "inaccessible code" error in the context of this Given!

For example, see typecheck/should_compile/T10493, repeated here:

  import Data.Ord (Down)  -- no constructor

  foo :: Coercible (Down Int) Int => Down Int -> Int
  foo = coerce

That should compile, but only because we use canEqFailure and not
canEqHardFailure.

Note [Decomposing equality]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have a constraint (of any flavour and role) that looks like
T tys1 ~ T tys2, what can we conclude about tys1 and tys2? The answer,
of course, is "it depends". This Note spells it all out.

In this Note, "decomposition" refers to taking the constraint
  [fl] (T tys1 ~X T tys2)
(for some flavour fl and some role X) and replacing it with
  [fls'] (tys1 ~Xs' tys2)
where that notation indicates a list of new constraints, where the
new constraints may have different flavours and different roles.

The key property to consider is injectivity. When decomposing a Given, the
decomposition is sound if and only if T is injective in all of its type
arguments. When decomposing a Wanted, the decomposition is sound (assuming the
correct roles in the produced equality constraints), but it may be a guess --
that is, an unforced decision by the constraint solver. Decomposing Wanteds
over injective TyCons does not entail guessing. But sometimes we want to
decompose a Wanted even when the TyCon involved is not injective! (See below.)

So, in broad strokes, we want this rule:

(*) Decompose a constraint (T tys1 ~X T tys2) if and only if T is injective
at role X.

Pursuing the details requires exploring three axes:
* Flavour: Given vs. Derived vs. Wanted
* Role: Nominal vs. Representational
* TyCon species: datatype vs. newtype vs. data family vs. type family vs. type variable

(A type variable isn't a TyCon, of course, but it's convenient to put the AppTy case
in the same table.)

Right away, we can say that Derived behaves just as Wanted for the purposes
of decomposition. The difference between Derived and Wanted is the handling of
evidence. Since decomposition in these cases isn't a matter of soundness but of
guessing, we want the same behaviour regardless of evidence.

Here is a table (discussion following) detailing where decomposition of
   (T s1 ... sn) ~r (T t1 .. tn)
is allowed.  The first four lines (Data types ... type family) refer
to TyConApps with various TyCons T; the last line is for AppTy, covering
both where there is a type variable at the head and the case for an over-
saturated type family.

NOMINAL               GIVEN        WANTED                         WHERE

Datatype               YES          YES                           canTyConApp
Newtype                YES          YES                           canTyConApp
Data family            YES          YES                           canTyConApp
Type family            NO{1}        YES, in injective args{1}     canEqCanLHS2
AppTy                  YES          YES                           can_eq_app

REPRESENTATIONAL      GIVEN        WANTED

Datatype               YES          YES                           canTyConApp
Newtype                NO{2}       MAYBE{2}                canTyConApp(can_decompose)
Data family            NO{3}       MAYBE{3}                canTyConApp(can_decompose)
Type family            NO           NO                            canEqCanLHS2
AppTy                  NO{4}        NO{4}                         can_eq_nc'

{1}: Type families can be injective in some, but not all, of their arguments,
so we want to do partial decomposition. This is quite different than the way
other decomposition is done, where the decomposed equalities replace the original
one. We thus proceed much like we do with superclasses, emitting new Deriveds
when "decomposing" a partially-injective type family Wanted. Injective type
families have no corresponding evidence of their injectivity, so we cannot
decompose an injective-type-family Given.

{2}: See Note [Decomposing newtypes at representational role]

{3}: Because of the possibility of newtype instances, we must treat
data families like newtypes. See also
Note [Decomposing newtypes at representational role]. See #10534 and
test case typecheck/should_fail/T10534.

{4}: See Note [Decomposing AppTy at representational role]

In the implementation of can_eq_nc and friends, we don't directly pattern
match using lines like in the tables above, as those tables don't cover
all cases (what about PrimTyCon? tuples?). Instead we just ask about injectivity,
boiling the tables above down to rule (*). The exceptions to rule (*) are for
injective type families, which are handled separately from other decompositions,
and the MAYBE entries above.

Note [Decomposing newtypes at representational role]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This note discusses the 'newtype' line in the REPRESENTATIONAL table
in Note [Decomposing equality]. (At nominal role, newtypes are fully
decomposable.)

Here is a representative example of why representational equality over
newtypes is tricky:

  newtype Nt a = Mk Bool         -- NB: a is not used in the RHS,
  type role Nt representational  -- but the user gives it an R role anyway

If we have [W] Nt alpha ~R Nt beta, we *don't* want to decompose to
[W] alpha ~R beta, because it's possible that alpha and beta aren't
representationally equal. Here's another example.

  newtype Nt a = MkNt (Id a)
  type family Id a where Id a = a

  [W] Nt Int ~R Nt Age

Because of its use of a type family, Nt's parameter will get inferred to have
a nominal role. Thus, decomposing the wanted will yield [W] Int ~N Age, which
is unsatisfiable. Unwrapping, though, leads to a solution.

Conclusion:
 * Unwrap newtypes before attempting to decompose them.
   This is done in can_eq_nc'.

It all comes from the fact that newtypes aren't necessarily injective
w.r.t. representational equality.

Furthermore, as explained in Note [NthCo and newtypes] in GHC.Core.TyCo.Rep, we can't use
NthCo on representational coercions over newtypes. NthCo comes into play
only when decomposing givens.

Conclusion:
 * Do not decompose [G] N s ~R N t

Is it sensible to decompose *Wanted* constraints over newtypes?  Yes!
It's the only way we could ever prove (IO Int ~R IO Age), recalling
that IO is a newtype.

However we must be careful.  Consider

  type role Nt representational

  [G] Nt a ~R Nt b       (1)
  [W] NT alpha ~R Nt b   (2)
  [W] alpha ~ a          (3)

If we focus on (3) first, we'll substitute in (2), and now it's
identical to the given (1), so we succeed.  But if we focus on (2)
first, and decompose it, we'll get (alpha ~R b), which is not soluble.
This is exactly like the question of overlapping Givens for class
constraints: see Note [Instance and Given overlap] in GHC.Tc.Solver.Interact.

Conclusion:
  * Decompose [W] N s ~R N t  iff there no given constraint that could
    later solve it.

Note [Decomposing AppTy at representational role]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We never decompose AppTy at a representational role. For Givens, doing
so is simply unsound: the LRCo coercion former requires a nominal-roled
arguments. (See (1) for an example of why.) For Wanteds, decomposing
would be sound, but it would be a guess, and a non-confluent one at that.

Here is an example:

    [G] g1 :: a ~R b
    [W] w1 :: Maybe b ~R alpha a
    [W] w2 :: alpha ~ Maybe

Suppose we see w1 before w2. If we were to decompose, we would decompose
this to become

    [W] w3 :: Maybe ~R alpha
    [W] w4 :: b ~ a

Note that w4 is *nominal*. A nominal role here is necessary because AppCo
requires a nominal role on its second argument. (See (2) for an example of
why.) If we decomposed w1 to w3,w4, we would then get stuck, because w4
is insoluble. On the other hand, if we see w2 first, setting alpha := Maybe,
all is well, as we can decompose Maybe b ~R Maybe a into b ~R a.

Another example:

    newtype Phant x = MkPhant Int

    [W] w1 :: Phant Int ~R alpha Bool
    [W] w2 :: alpha ~ Phant

If we see w1 first, decomposing would be disastrous, as we would then try
to solve Int ~ Bool. Instead, spotting w2 allows us to simplify w1 to become

    [W] w1' :: Phant Int ~R Phant Bool

which can then (assuming MkPhant is in scope) be simplified to Int ~R Int,
and all will be well. See also Note [Unwrap newtypes first].

Bottom line: never decompose AppTy with representational roles.

(1) Decomposing a Given AppTy over a representational role is simply
unsound. For example, if we have co1 :: Phant Int ~R a Bool (for
the newtype Phant, above), then we surely don't want any relationship
between Int and Bool, lest we also have co2 :: Phant ~ a around.

(2) The role on the AppCo coercion is a conservative choice, because we don't
know the role signature of the function. For example, let's assume we could
have a representational role on the second argument of AppCo. Then, consider

    data G a where    -- G will have a nominal role, as G is a GADT
      MkG :: G Int
    newtype Age = MkAge Int

    co1 :: G ~R a        -- by assumption
    co2 :: Age ~R Int    -- by newtype axiom
    co3 = AppCo co1 co2 :: G Age ~R a Int    -- by our broken AppCo

and now co3 can be used to cast MkG to have type G Age, in violation of
the way GADTs are supposed to work (which is to use nominal equality).

-}

canDecomposableTyConAppOK :: CtEvidence -> EqRel
                          -> TyCon -> [TcType] -> [TcType]
                          -> TcS (StopOrContinue Ct)
-- Precondition: tys1 and tys2 are the same length, hence "OK"
canDecomposableTyConAppOK :: CtEvidence
-> EqRel -> TyCon -> [Xi] -> [Xi] -> TcS (StopOrContinue Ct)
canDecomposableTyConAppOK CtEvidence
ev EqRel
eq_rel TyCon
tc [Xi]
tys1 [Xi]
tys2
  = ASSERT( tys1 `equalLength` tys2 )
    do { String -> SDoc -> TcS ()
traceTcS String
"canDecomposableTyConAppOK"
                  (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev SDoc -> SDoc -> SDoc
$$ EqRel -> SDoc
forall a. Outputable a => a -> SDoc
ppr EqRel
eq_rel SDoc -> SDoc -> SDoc
$$ TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
tc SDoc -> SDoc -> SDoc
$$ [Xi] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Xi]
tys1 SDoc -> SDoc -> SDoc
$$ [Xi] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Xi]
tys2)
       ; case CtEvidence
ev of
           CtDerived {}
             -> CtLoc -> [Role] -> [Xi] -> [Xi] -> TcS ()
unifyDeriveds CtLoc
loc [Role]
tc_roles [Xi]
tys1 [Xi]
tys2

           CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest }
                  -- new_locs and tc_roles are both infinite, so
                  -- we are guaranteed that cos has the same length
                  -- as tys1 and tys2
             -> do { [TcCoercion]
cos <- (CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion)
-> [CtLoc] -> [Role] -> [Xi] -> [Xi] -> TcS [TcCoercion]
forall (m :: * -> *) a b c d e.
Monad m =>
(a -> b -> c -> d -> m e) -> [a] -> [b] -> [c] -> [d] -> m [e]
zipWith4M CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted [CtLoc]
new_locs [Role]
tc_roles [Xi]
tys1 [Xi]
tys2
                   ; TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest (HasDebugCallStack => Role -> TyCon -> [TcCoercion] -> TcCoercion
Role -> TyCon -> [TcCoercion] -> TcCoercion
mkTyConAppCo Role
role TyCon
tc [TcCoercion]
cos) }

           CtGiven { ctev_evar :: CtEvidence -> TyVar
ctev_evar = TyVar
evar }
             -> do { let ev_co :: TcCoercion
ev_co = TyVar -> TcCoercion
mkCoVarCo TyVar
evar
                   ; [CtEvidence]
given_evs <- CtLoc -> [(Xi, EvTerm)] -> TcS [CtEvidence]
newGivenEvVars CtLoc
loc ([(Xi, EvTerm)] -> TcS [CtEvidence])
-> [(Xi, EvTerm)] -> TcS [CtEvidence]
forall a b. (a -> b) -> a -> b
$
                                  [ ( Role -> Xi -> Xi -> Xi
mkPrimEqPredRole Role
r Xi
ty1 Xi
ty2
                                    , TcCoercion -> EvTerm
evCoercion (TcCoercion -> EvTerm) -> TcCoercion -> EvTerm
forall a b. (a -> b) -> a -> b
$ HasDebugCallStack => Role -> Int -> TcCoercion -> TcCoercion
Role -> Int -> TcCoercion -> TcCoercion
mkNthCo Role
r Int
i TcCoercion
ev_co )
                                  | (Role
r, Xi
ty1, Xi
ty2, Int
i) <- [Role] -> [Xi] -> [Xi] -> [Int] -> [(Role, Xi, Xi, Int)]
forall a b c d. [a] -> [b] -> [c] -> [d] -> [(a, b, c, d)]
zip4 [Role]
tc_roles [Xi]
tys1 [Xi]
tys2 [Int
0..]
                                  , Role
r Role -> Role -> Bool
forall a. Eq a => a -> a -> Bool
/= Role
Phantom
                                  , Bool -> Bool
not (Xi -> Bool
isCoercionTy Xi
ty1) Bool -> Bool -> Bool
&& Bool -> Bool
not (Xi -> Bool
isCoercionTy Xi
ty2) ]
                   ; [CtEvidence] -> TcS ()
emitWorkNC [CtEvidence]
given_evs }

    ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Decomposed TyConApp" }

  where
    loc :: CtLoc
loc        = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    role :: Role
role       = EqRel -> Role
eqRelRole EqRel
eq_rel

      -- infinite, as tyConRolesX returns an infinite tail of Nominal
    tc_roles :: [Role]
tc_roles   = Role -> TyCon -> [Role]
tyConRolesX Role
role TyCon
tc

      -- Add nuances to the location during decomposition:
      --  * if the argument is a kind argument, remember this, so that error
      --    messages say "kind", not "type". This is determined based on whether
      --    the corresponding tyConBinder is named (that is, dependent)
      --  * if the argument is invisible, note this as well, again by
      --    looking at the corresponding binder
      -- For oversaturated tycons, we need the (repeat loc) tail, which doesn't
      -- do either of these changes. (Forgetting to do so led to #16188)
      --
      -- NB: infinite in length
    new_locs :: [CtLoc]
new_locs = [ CtLoc
new_loc
               | TyConBinder
bndr <- TyCon -> [TyConBinder]
tyConBinders TyCon
tc
               , let new_loc0 :: CtLoc
new_loc0 | TyConBinder -> Bool
isNamedTyConBinder TyConBinder
bndr = CtLoc -> CtLoc
toKindLoc CtLoc
loc
                              | Bool
otherwise               = CtLoc
loc
                     new_loc :: CtLoc
new_loc  | TyConBinder -> Bool
forall tv. VarBndr tv TyConBndrVis -> Bool
isInvisibleTyConBinder TyConBinder
bndr
                              = CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc
updateCtLocOrigin CtLoc
new_loc0 CtOrigin -> CtOrigin
toInvisibleOrigin
                              | Bool
otherwise
                              = CtLoc
new_loc0 ]
               [CtLoc] -> [CtLoc] -> [CtLoc]
forall a. [a] -> [a] -> [a]
++ CtLoc -> [CtLoc]
forall a. a -> [a]
repeat CtLoc
loc

-- | Call when canonicalizing an equality fails, but if the equality is
-- representational, there is some hope for the future.
-- Examples in Note [Use canEqFailure in canDecomposableTyConApp]
canEqFailure :: CtEvidence -> EqRel
             -> TcType -> TcType -> TcS (StopOrContinue Ct)
canEqFailure :: CtEvidence -> EqRel -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqFailure CtEvidence
ev EqRel
NomEq Xi
ty1 Xi
ty2
  = CtEvidence -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev Xi
ty1 Xi
ty2
canEqFailure CtEvidence
ev EqRel
ReprEq Xi
ty1 Xi
ty2
  = do { (Xi
xi1, TcCoercion
co1) <- CtEvidence -> Xi -> TcS (Xi, TcCoercion)
rewrite CtEvidence
ev Xi
ty1
       ; (Xi
xi2, TcCoercion
co2) <- CtEvidence -> Xi -> TcS (Xi, TcCoercion)
rewrite CtEvidence
ev Xi
ty2
            -- We must rewrite the types before putting them in the
            -- inert set, so that we are sure to kick them out when
            -- new equalities become available
       ; String -> SDoc -> TcS ()
traceTcS String
"canEqFailure with ReprEq" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
vcat [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty1, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty2, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
xi1, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
xi2 ]
       ; CtEvidence
new_ev <- CtEvidence
-> SwapFlag
-> Xi
-> Xi
-> TcCoercion
-> TcCoercion
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
NotSwapped Xi
xi1 Xi
xi2 TcCoercion
co1 TcCoercion
co2
       ; Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredStatus -> CtEvidence -> Ct
mkIrredCt CtIrredStatus
OtherCIS CtEvidence
new_ev) }

-- | Call when canonicalizing an equality fails with utterly no hope.
canEqHardFailure :: CtEvidence
                 -> TcType -> TcType -> TcS (StopOrContinue Ct)
-- See Note [Make sure that insolubles are fully rewritten]
canEqHardFailure :: CtEvidence -> Xi -> Xi -> TcS (StopOrContinue Ct)
canEqHardFailure CtEvidence
ev Xi
ty1 Xi
ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqHardFailure" (Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty1 SDoc -> SDoc -> SDoc
$$ Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ty2)
       ; (Xi
s1, TcCoercion
co1) <- CtEvidence -> Xi -> TcS (Xi, TcCoercion)
rewrite CtEvidence
ev Xi
ty1
       ; (Xi
s2, TcCoercion
co2) <- CtEvidence -> Xi -> TcS (Xi, TcCoercion)
rewrite CtEvidence
ev Xi
ty2
       ; CtEvidence
new_ev <- CtEvidence
-> SwapFlag
-> Xi
-> Xi
-> TcCoercion
-> TcCoercion
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
NotSwapped Xi
s1 Xi
s2 TcCoercion
co1 TcCoercion
co2
       ; Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredStatus -> CtEvidence -> Ct
mkIrredCt CtIrredStatus
InsolubleCIS CtEvidence
new_ev) }

{-
Note [Decomposing TyConApps]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we see (T s1 t1 ~ T s2 t2), then we can just decompose to
  (s1 ~ s2, t1 ~ t2)
and push those back into the work list.  But if
  s1 = K k1    s2 = K k2
then we will just decomopose s1~s2, and it might be better to
do so on the spot.  An important special case is where s1=s2,
and we get just Refl.

So canDecomposableTyCon is a fast-path decomposition that uses
unifyWanted etc to short-cut that work.

Note [Canonicalising type applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Given (s1 t1) ~ ty2, how should we proceed?
The simple thing is to see if ty2 is of form (s2 t2), and
decompose.

However, over-eager decomposition gives bad error messages
for things like
   a b ~ Maybe c
   e f ~ p -> q
Suppose (in the first example) we already know a~Array.  Then if we
decompose the application eagerly, yielding
   a ~ Maybe
   b ~ c
we get an error        "Can't match Array ~ Maybe",
but we'd prefer to get "Can't match Array b ~ Maybe c".

So instead can_eq_wanted_app rewrites the LHS and RHS, in the hope of
replacing (a b) by (Array b), before using try_decompose_app to
decompose it.

Note [Make sure that insolubles are fully rewritten]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When an equality fails, we still want to rewrite the equality
all the way down, so that it accurately reflects
 (a) the mutable reference substitution in force at start of solving
 (b) any ty-binds in force at this point in solving
See Note [Rewrite insolubles] in GHC.Tc.Solver.Monad.
And if we don't do this there is a bad danger that
GHC.Tc.Solver.applyTyVarDefaulting will find a variable
that has in fact been substituted.

Note [Do not decompose Given polytype equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider [G] (forall a. t1 ~ forall a. t2).  Can we decompose this?
No -- what would the evidence look like?  So instead we simply discard
this given evidence.


Note [Combining insoluble constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As this point we have an insoluble constraint, like Int~Bool.

 * If it is Wanted, delete it from the cache, so that subsequent
   Int~Bool constraints give rise to separate error messages

 * But if it is Derived, DO NOT delete from cache.  A class constraint
   may get kicked out of the inert set, and then have its functional
   dependency Derived constraints generated a second time. In that
   case we don't want to get two (or more) error messages by
   generating two (or more) insoluble fundep constraints from the same
   class constraint.

Note [No top-level newtypes on RHS of representational equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we're in this situation:

 work item:  [W] c1 : a ~R b
     inert:  [G] c2 : b ~R Id a

where
  newtype Id a = Id a

We want to make sure canEqCanLHS sees [W] a ~R a, after b is rewritten
and the Id newtype is unwrapped. This is assured by requiring only rewritten
types in canEqCanLHS *and* having the newtype-unwrapping check above
the tyvar check in can_eq_nc.

Note [Occurs check error]
~~~~~~~~~~~~~~~~~~~~~~~~~
If we have an occurs check error, are we necessarily hosed? Say our
tyvar is tv1 and the type it appears in is xi2. Because xi2 is function
free, then if we're computing w.r.t. nominal equality, then, yes, we're
hosed. Nothing good can come from (a ~ [a]). If we're computing w.r.t.
representational equality, this is a little subtler. Once again, (a ~R [a])
is a bad thing, but (a ~R N a) for a newtype N might be just fine. This
means also that (a ~ b a) might be fine, because `b` might become a newtype.

So, we must check: does tv1 appear in xi2 under any type constructor
that is generative w.r.t. representational equality? That's what
isInsolubleOccursCheck does.

See also #10715, which induced this addition.

Note [Put touchable variables on the left]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ticket #10009, a very nasty example:

    f :: (UnF (F b) ~ b) => F b -> ()

    g :: forall a. (UnF (F a) ~ a) => a -> ()
    g _ = f (undefined :: F a)

For g we get [G]  g1 : UnF (F a) ~ a
             [WD] w1 : UnF (F beta) ~ beta
             [WD] w2 : F a ~ F beta

g1 is canonical (CEqCan). It is oriented as above because a is not touchable.
See canEqTyVarFunEq.

w1 is similarly canonical, though the occurs-check in canEqTyVarFunEq is key
here.

w2 is canonical. But which way should it be oriented? As written, we'll be
stuck. When w2 is added to the inert set, nothing gets kicked out: g1 is
a Given (and Wanteds don't rewrite Givens), and w2 doesn't mention the LHS
of w2. We'll thus lose.

But if w2 is swapped around, to

    [D] w3 : F beta ~ F a

then (after emitting shadow Deriveds, etc. See GHC.Tc.Solver.Monad
Note [The improvement story and derived shadows]) we'll kick w1 out of the inert
set (it mentions the LHS of w3). We then rewrite w1 to

    [D] w4 : UnF (F a) ~ beta

and then, using g1, to

    [D] w5 : a ~ beta

at which point we can unify and go on to glory. (This rewriting actually
happens all at once, in the call to rewrite during canonicalisation.)

But what about the new LHS makes it better? It mentions a variable (beta)
that can appear in a Wanted -- a touchable metavariable never appears
in a Given. On the other hand, the original LHS mentioned only variables
that appear in Givens. We thus choose to put variables that can appear
in Wanteds on the left.

Ticket #12526 is another good example of this in action.

-}

---------------------
canEqCanLHS :: CtEvidence          -- ev :: lhs ~ rhs
            -> EqRel -> SwapFlag
            -> CanEqLHS              -- lhs (or, if swapped, rhs)
            -> TcType                -- lhs: pretty lhs, already rewritten
            -> TcType -> TcType      -- rhs: already rewritten
            -> TcS (StopOrContinue Ct)
canEqCanLHS :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Xi
ps_xi1 Xi
xi2 Xi
ps_xi2
  | Xi
k1 HasDebugCallStack => Xi -> Xi -> Bool
Xi -> Xi -> Bool
`tcEqType` Xi
k2
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCanLHSHomo CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Xi
ps_xi1 Xi
xi2 Xi
ps_xi2

  | Bool
otherwise
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCanLHSHetero CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Xi
ps_xi1 Xi
k1 Xi
xi2 Xi
ps_xi2 Xi
k2

  where
    k1 :: Xi
k1 = CanEqLHS -> Xi
canEqLHSKind CanEqLHS
lhs1
    k2 :: Xi
k2 = HasDebugCallStack => Xi -> Xi
Xi -> Xi
tcTypeKind Xi
xi2

canEqCanLHSHetero :: CtEvidence         -- :: (xi1 :: ki1) ~ (xi2 :: ki2)
                  -> EqRel -> SwapFlag
                  -> CanEqLHS -> TcType -- xi1, pretty xi1
                  -> TcKind             -- ki1
                  -> TcType -> TcType   -- xi2, pretty xi2 :: ki2
                  -> TcKind             -- ki2
                  -> TcS (StopOrContinue Ct)
canEqCanLHSHetero :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> Xi
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCanLHSHetero CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Xi
ps_xi1 Xi
ki1 Xi
xi2 Xi
ps_xi2 Xi
ki2
  -- See Note [Equalities with incompatible kinds]
  = do { TcCoercion
kind_co <- TcS TcCoercion
emit_kind_co   -- :: ki2 ~N ki1

       ; let  -- kind_co :: (ki2 :: *) ~N (ki1 :: *)   (whether swapped or not)
              -- co1     :: kind(tv1) ~N ki1
             rhs' :: Xi
rhs'    = Xi
xi2    Xi -> TcCoercion -> Xi
`mkCastTy` TcCoercion
kind_co   -- :: ki1
             ps_rhs' :: Xi
ps_rhs' = Xi
ps_xi2 Xi -> TcCoercion -> Xi
`mkCastTy` TcCoercion
kind_co   -- :: ki1
             rhs_co :: TcCoercion
rhs_co  = Role -> Xi -> TcCoercion -> TcCoercion
mkTcGReflLeftCo Role
role Xi
xi2 TcCoercion
kind_co
               -- rhs_co :: (xi2 |> kind_co) ~ xi2

             lhs_co :: TcCoercion
lhs_co = Role -> Xi -> TcCoercion
mkTcReflCo Role
role Xi
xi1

       ; String -> SDoc -> TcS ()
traceTcS String
"Hetero equality gives rise to kind equality"
           (TcCoercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcCoercion
kind_co SDoc -> SDoc -> SDoc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
<+> [SDoc] -> SDoc
sep [ Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ki2, String -> SDoc
text String
"~#", Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
ki1 ])
       ; CtEvidence
type_ev <- CtEvidence
-> SwapFlag
-> Xi
-> Xi
-> TcCoercion
-> TcCoercion
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped Xi
xi1 Xi
rhs' TcCoercion
lhs_co TcCoercion
rhs_co

          -- rewriteEqEvidence carries out the swap, so we're NotSwapped any more
       ; CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCanLHSHomo CtEvidence
type_ev EqRel
eq_rel SwapFlag
NotSwapped CanEqLHS
lhs1 Xi
ps_xi1 Xi
rhs' Xi
ps_rhs' }
  where
    emit_kind_co :: TcS CoercionN
    emit_kind_co :: TcS TcCoercion
emit_kind_co
      | CtGiven { ctev_evar :: CtEvidence -> TyVar
ctev_evar = TyVar
evar } <- CtEvidence
ev
      = do { let kind_co :: TcCoercion
kind_co = TcCoercion -> TcCoercion
maybe_sym (TcCoercion -> TcCoercion) -> TcCoercion -> TcCoercion
forall a b. (a -> b) -> a -> b
$ TcCoercion -> TcCoercion
mkTcKindCo (TyVar -> TcCoercion
mkTcCoVarCo TyVar
evar)  -- :: k2 ~ k1
           ; CtEvidence
kind_ev <- CtLoc -> (Xi, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
kind_loc (Xi
kind_pty, TcCoercion -> EvTerm
evCoercion TcCoercion
kind_co)
           ; [CtEvidence] -> TcS ()
emitWorkNC [CtEvidence
kind_ev]
           ; TcCoercion -> TcS TcCoercion
forall (m :: * -> *) a. Monad m => a -> m a
return (HasDebugCallStack => CtEvidence -> TcCoercion
CtEvidence -> TcCoercion
ctEvCoercion CtEvidence
kind_ev) }

      | Bool
otherwise
      = CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted CtLoc
kind_loc Role
Nominal Xi
ki2 Xi
ki1

    xi1 :: Xi
xi1      = CanEqLHS -> Xi
canEqLHSType CanEqLHS
lhs1
    loc :: CtLoc
loc      = CtEvidence -> CtLoc
ctev_loc CtEvidence
ev
    role :: Role
role     = EqRel -> Role
eqRelRole EqRel
eq_rel
    kind_loc :: CtLoc
kind_loc = Xi -> Xi -> CtLoc -> CtLoc
mkKindLoc Xi
xi1 Xi
xi2 CtLoc
loc
    kind_pty :: Xi
kind_pty = Xi -> Xi -> Xi -> Xi -> Xi
mkHeteroPrimEqPred Xi
liftedTypeKind Xi
liftedTypeKind Xi
ki2 Xi
ki1

    maybe_sym :: TcCoercion -> TcCoercion
maybe_sym = case SwapFlag
swapped of
          SwapFlag
IsSwapped  -> TcCoercion -> TcCoercion
forall a. a -> a
id         -- if the input is swapped, then we already
                                   -- will have k2 ~ k1
          SwapFlag
NotSwapped -> TcCoercion -> TcCoercion
mkTcSymCo

-- guaranteed that tcTypeKind lhs == tcTypeKind rhs
canEqCanLHSHomo :: CtEvidence
                -> EqRel -> SwapFlag
                -> CanEqLHS           -- lhs (or, if swapped, rhs)
                -> TcType             -- pretty lhs
                -> TcType -> TcType   -- rhs, pretty rhs
                -> TcS (StopOrContinue Ct)
canEqCanLHSHomo :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> Xi
-> Xi
-> TcS (StopOrContinue Ct)
canEqCanLHSHomo CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Xi
ps_xi1 Xi
xi2 Xi
ps_xi2
  | (Xi
xi2', MCoercion
mco) <- Xi -> (Xi, MCoercion)
split_cast_ty Xi
xi2
  , Just CanEqLHS
lhs2 <- Xi -> Maybe CanEqLHS
canEqLHS_maybe Xi
xi2'
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> CanEqLHS
-> Xi
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqCanLHS2 CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Xi
ps_xi1 CanEqLHS
lhs2 (Xi
ps_xi2 Xi -> MCoercion -> Xi
`mkCastTyMCo` MCoercion -> MCoercion
mkTcSymMCo MCoercion
mco) MCoercion
mco

  | Bool
otherwise
  = CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Xi -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Xi
ps_xi2

  where
    split_cast_ty :: Xi -> (Xi, MCoercion)
split_cast_ty (CastTy Xi
ty TcCoercion
co) = (Xi
ty, TcCoercion -> MCoercion
MCo TcCoercion
co)
    split_cast_ty Xi
other          = (Xi
other, MCoercion
MRefl)

-- This function deals with the case that both LHS and RHS are potential
-- CanEqLHSs.
canEqCanLHS2 :: CtEvidence              -- lhs ~ (rhs |> mco)
                                        -- or, if swapped: (rhs |> mco) ~ lhs
             -> EqRel -> SwapFlag
             -> CanEqLHS                -- lhs (or, if swapped, rhs)
             -> TcType                  -- pretty lhs
             -> CanEqLHS                -- rhs
             -> TcType                  -- pretty rhs
             -> MCoercion               -- :: kind(rhs) ~N kind(lhs)
             -> TcS (StopOrContinue Ct)
canEqCanLHS2 :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Xi
-> CanEqLHS
-> Xi
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqCanLHS2 CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Xi
ps_xi1 CanEqLHS
lhs2 Xi
ps_xi2 MCoercion
mco
  | CanEqLHS
lhs1 CanEqLHS -> CanEqLHS -> Bool
`eqCanEqLHS` CanEqLHS
lhs2
    -- It must be the case that mco is reflexive
  = CtEvidence -> EqRel -> Xi -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
eq_rel (CanEqLHS -> Xi
canEqLHSType CanEqLHS
lhs1)

  | TyVarLHS TyVar
tv1 <- CanEqLHS
lhs1
  , TyVarLHS TyVar
tv2 <- CanEqLHS
lhs2
  , Bool -> TyVar -> TyVar -> Bool
swapOverTyVars (CtEvidence -> Bool
isGiven CtEvidence
ev) TyVar
tv1 TyVar
tv2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqLHS2 swapOver" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv1 SDoc -> SDoc -> SDoc
$$ TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv2 SDoc -> SDoc -> SDoc
$$ SwapFlag -> SDoc
forall a. Outputable a => a -> SDoc
ppr SwapFlag
swapped)
       ; CtEvidence
new_ev <- TcS CtEvidence
do_swap
       ; CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Xi -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
new_ev EqRel
eq_rel SwapFlag
IsSwapped (TyVar -> CanEqLHS
TyVarLHS TyVar
tv2)
                                                   (Xi
ps_xi1 Xi -> MCoercion -> Xi
`mkCastTyMCo` MCoercion
sym_mco) }

  | TyVarLHS TyVar
tv1 <- CanEqLHS
lhs1
  , TyFamLHS TyCon
fun_tc2 [Xi]
fun_args2 <- CanEqLHS
lhs2
  = CtEvidence
-> EqRel
-> SwapFlag
-> TyVar
-> Xi
-> TyCon
-> [Xi]
-> Xi
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqTyVarFunEq CtEvidence
ev EqRel
eq_rel SwapFlag
swapped TyVar
tv1 Xi
ps_xi1 TyCon
fun_tc2 [Xi]
fun_args2 Xi
ps_xi2 MCoercion
mco

  | TyFamLHS TyCon
fun_tc1 [Xi]
fun_args1 <- CanEqLHS
lhs1
  , TyVarLHS TyVar
tv2 <- CanEqLHS
lhs2
  = do { CtEvidence
new_ev <- TcS CtEvidence
do_swap
       ; CtEvidence
-> EqRel
-> SwapFlag
-> TyVar
-> Xi
-> TyCon
-> [Xi]
-> Xi
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqTyVarFunEq CtEvidence
new_ev EqRel
eq_rel SwapFlag
IsSwapped TyVar
tv2 Xi
ps_xi2
                                                 TyCon
fun_tc1 [Xi]
fun_args1 Xi
ps_xi1 MCoercion
sym_mco }

  | TyFamLHS TyCon
fun_tc1 [Xi]
fun_args1 <- CanEqLHS
lhs1
  , TyFamLHS TyCon
fun_tc2 [Xi]
fun_args2 <- CanEqLHS
lhs2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqCanLHS2 two type families" (CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs1 SDoc -> SDoc -> SDoc
$$ CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs2)

         -- emit derived equalities for injective type families
       ; let inj_eqns :: [TypeEqn]  -- TypeEqn = Pair Type
             inj_eqns :: [Pair Xi]
inj_eqns
               | EqRel
ReprEq <- EqRel
eq_rel   = []   -- injectivity applies only for nom. eqs.
               | TyCon
fun_tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
/= TyCon
fun_tc2 = []   -- if the families don't match, stop.

               | Injective [Bool]
inj <- TyCon -> Injectivity
tyConInjectivityInfo TyCon
fun_tc1
               = [ Xi -> Xi -> Pair Xi
forall a. a -> a -> Pair a
Pair Xi
arg1 Xi
arg2
                 | (Xi
arg1, Xi
arg2, Bool
True) <- [Xi] -> [Xi] -> [Bool] -> [(Xi, Xi, Bool)]
forall a b c. [a] -> [b] -> [c] -> [(a, b, c)]
zip3 [Xi]
fun_args1 [Xi]
fun_args2 [Bool]
inj ]

                 -- built-in synonym families don't have an entry point
                 -- for this use case. So, we just use sfInteractInert
                 -- and pass two equal RHSs. We *could* add another entry
                 -- point, but then there would be a burden to make
                 -- sure the new entry point and existing ones were
                 -- internally consistent. This is slightly distasteful,
                 -- but it works well in practice and localises the
                 -- problem.
               | Just BuiltInSynFamily
ops <- TyCon -> Maybe BuiltInSynFamily
isBuiltInSynFamTyCon_maybe TyCon
fun_tc1
               = let ki1 :: Xi
ki1 = CanEqLHS -> Xi
canEqLHSKind CanEqLHS
lhs1
                     ki2 :: Xi
ki2 | MCoercion
MRefl <- MCoercion
mco
                         = Xi
ki1   -- just a small optimisation
                         | Bool
otherwise
                         = CanEqLHS -> Xi
canEqLHSKind CanEqLHS
lhs2

                     fake_rhs1 :: Xi
fake_rhs1 = Xi -> Xi
anyTypeOfKind Xi
ki1
                     fake_rhs2 :: Xi
fake_rhs2 = Xi -> Xi
anyTypeOfKind Xi
ki2
                 in
                 BuiltInSynFamily -> [Xi] -> Xi -> [Xi] -> Xi -> [Pair Xi]
sfInteractInert BuiltInSynFamily
ops [Xi]
fun_args1 Xi
fake_rhs1 [Xi]
fun_args2 Xi
fake_rhs2

               | Bool
otherwise  -- ordinary, non-injective type family
               = []

       ; Bool -> TcS () -> TcS ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
unless (CtEvidence -> Bool
isGiven CtEvidence
ev) (TcS () -> TcS ()) -> TcS () -> TcS ()
forall a b. (a -> b) -> a -> b
$
         (Pair Xi -> TcS ()) -> [Pair Xi] -> TcS ()
forall (t :: * -> *) (m :: * -> *) a b.
(Foldable t, Monad m) =>
(a -> m b) -> t a -> m ()
mapM_ (CtLoc -> Role -> Pair Xi -> TcS ()
unifyDerived (CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev) Role
Nominal) [Pair Xi]
inj_eqns

       ; TcLevel
tclvl <- TcS TcLevel
getTcLevel
       ; DynFlags
dflags <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; let tvs1 :: VarSet
tvs1 = [Xi] -> VarSet
tyCoVarsOfTypes [Xi]
fun_args1
             tvs2 :: VarSet
tvs2 = [Xi] -> VarSet
tyCoVarsOfTypes [Xi]
fun_args2

             swap_for_rewriting :: Bool
swap_for_rewriting = (TyVar -> Bool) -> VarSet -> Bool
anyVarSet (TcLevel -> TyVar -> Bool
isTouchableMetaTyVar TcLevel
tclvl) VarSet
tvs2 Bool -> Bool -> Bool
&&
                          -- swap 'em: Note [Put touchable variables on the left]
                                  Bool -> Bool
not ((TyVar -> Bool) -> VarSet -> Bool
anyVarSet (TcLevel -> TyVar -> Bool
isTouchableMetaTyVar TcLevel
tclvl) VarSet
tvs1)
                          -- this check is just to avoid unfruitful swapping

               -- If we have F a ~ F (F a), we want to swap.
             swap_for_occurs :: Bool
swap_for_occurs
               | CheckTyEqResult
CTE_OK     <- DynFlags -> TyCon -> [Xi] -> Xi -> CheckTyEqResult
checkTyFamEq DynFlags
dflags TyCon
fun_tc2 [Xi]
fun_args2
                                            (TyCon -> [Xi] -> Xi
mkTyConApp TyCon
fun_tc1 [Xi]
fun_args1)
               , CheckTyEqResult
CTE_Occurs <- DynFlags -> TyCon -> [Xi] -> Xi -> CheckTyEqResult
checkTyFamEq DynFlags
dflags TyCon
fun_tc1 [Xi]
fun_args1
                                            (TyCon -> [Xi] -> Xi
mkTyConApp TyCon
fun_tc2 [Xi]
fun_args2)
               = Bool
True

               | Bool
otherwise
               = Bool
False

       ; if Bool
swap_for_rewriting Bool -> Bool -> Bool
|| Bool
swap_for_occurs
         then do { CtEvidence
new_ev <- TcS CtEvidence
do_swap
                 ; CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Xi -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
new_ev EqRel
eq_rel SwapFlag
IsSwapped CanEqLHS
lhs2 (Xi
ps_xi1 Xi -> MCoercion -> Xi
`mkCastTyMCo` MCoercion
sym_mco) }
         else TcS (StopOrContinue Ct)
finish_without_swapping }

  -- that's all the special cases. Now we just figure out which non-special case
  -- to continue to.
  | Bool
otherwise
  = TcS (StopOrContinue Ct)
finish_without_swapping

  where
    sym_mco :: MCoercion
sym_mco = MCoercion -> MCoercion
mkTcSymMCo MCoercion
mco

    do_swap :: TcS CtEvidence
do_swap = CtEvidence
-> EqRel -> SwapFlag -> Xi -> Xi -> MCoercion -> TcS CtEvidence
rewriteCastedEquality CtEvidence
ev EqRel
eq_rel SwapFlag
swapped (CanEqLHS -> Xi
canEqLHSType CanEqLHS
lhs1) (CanEqLHS -> Xi
canEqLHSType CanEqLHS
lhs2) MCoercion
mco
    finish_without_swapping :: TcS (StopOrContinue Ct)
finish_without_swapping = CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Xi -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 (Xi
ps_xi2 Xi -> MCoercion -> Xi
`mkCastTyMCo` MCoercion
mco)


-- This function handles the case where one side is a tyvar and the other is
-- a type family application. Which to put on the left?
--   If the tyvar is a touchable meta-tyvar, put it on the left, as this may
--   be our only shot to unify.
--   Otherwise, put the function on the left, because it's generally better to
--   rewrite away function calls. This makes types smaller. And it seems necessary:
--     [W] F alpha ~ alpha
--     [W] F alpha ~ beta
--     [W] G alpha beta ~ Int   ( where we have type instance G a a = a )
--   If we end up with a stuck alpha ~ F alpha, we won't be able to solve this.
--   Test case: indexed-types/should_compile/CEqCanOccursCheck
canEqTyVarFunEq :: CtEvidence               -- :: lhs ~ (rhs |> mco)
                                            -- or (rhs |> mco) ~ lhs if swapped
                -> EqRel -> SwapFlag
                -> TyVar -> TcType          -- lhs (or if swapped rhs), pretty lhs
                -> TyCon -> [Xi] -> TcType  -- rhs (or if swapped lhs) fun and args, pretty rhs
                -> MCoercion                -- :: kind(rhs) ~N kind(lhs)
                -> TcS (StopOrContinue Ct)
canEqTyVarFunEq :: CtEvidence
-> EqRel
-> SwapFlag
-> TyVar
-> Xi
-> TyCon
-> [Xi]
-> Xi
-> MCoercion
-> TcS (StopOrContinue Ct)
canEqTyVarFunEq CtEvidence
ev EqRel
eq_rel SwapFlag
swapped TyVar
tv1 Xi
ps_xi1 TyCon
fun_tc2 [Xi]
fun_args2 Xi
ps_xi2 MCoercion
mco
  = do { UnifyTestResult
can_unify <- CtEvidence -> TyVar -> Xi -> TcS UnifyTestResult
unifyTest CtEvidence
ev TyVar
tv1 Xi
rhs
       ; DynFlags
dflags    <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; if | case UnifyTestResult
can_unify of { UnifyTestResult
NoUnify -> Bool
False; UnifyTestResult
_ -> Bool
True }
            , CheckTyEqResult
CTE_OK <- DynFlags -> AreTypeFamiliesOK -> TyVar -> Xi -> CheckTyEqResult
checkTyVarEq DynFlags
dflags AreTypeFamiliesOK
YesTypeFamilies TyVar
tv1 Xi
rhs
            -> CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Xi -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped (TyVar -> CanEqLHS
TyVarLHS TyVar
tv1) Xi
rhs

            | Bool
otherwise
              -> do { CtEvidence
new_ev <- CtEvidence
-> EqRel -> SwapFlag -> Xi -> Xi -> MCoercion -> TcS CtEvidence
rewriteCastedEquality CtEvidence
ev EqRel
eq_rel SwapFlag
swapped
                                  (TyVar -> Xi
mkTyVarTy TyVar
tv1) (TyCon -> [Xi] -> Xi
mkTyConApp TyCon
fun_tc2 [Xi]
fun_args2)
                                  MCoercion
mco
                    ; CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Xi -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
new_ev EqRel
eq_rel SwapFlag
IsSwapped
                                  (TyCon -> [Xi] -> CanEqLHS
TyFamLHS TyCon
fun_tc2 [Xi]
fun_args2)
                                  (Xi
ps_xi1 Xi -> MCoercion -> Xi
`mkCastTyMCo` MCoercion
sym_mco) } }
  where
    sym_mco :: MCoercion
sym_mco = MCoercion -> MCoercion
mkTcSymMCo MCoercion
mco
    rhs :: Xi
rhs = Xi
ps_xi2 Xi -> MCoercion -> Xi
`mkCastTyMCo` MCoercion
mco

data UnifyTestResult
  -- See Note [Solve by unification] in GHC.Tc.Solver.Interact
  -- which points out that having UnifySameLevel is just an optimisation;
  -- we could manage with UnifyOuterLevel alone (suitably renamed)
  = UnifySameLevel
  | UnifyOuterLevel [TcTyVar]   -- Promote these
                    TcLevel     -- ..to this level
  | NoUnify

instance Outputable UnifyTestResult where
  ppr :: UnifyTestResult -> SDoc
ppr UnifyTestResult
UnifySameLevel            = String -> SDoc
text String
"UnifySameLevel"
  ppr (UnifyOuterLevel [TyVar]
tvs TcLevel
lvl) = String -> SDoc
text String
"UnifyOuterLevel" SDoc -> SDoc -> SDoc
<> SDoc -> SDoc
parens (TcLevel -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcLevel
lvl SDoc -> SDoc -> SDoc
<+> [TyVar] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TyVar]
tvs)
  ppr UnifyTestResult
NoUnify                   = String -> SDoc
text String
"NoUnify"

unifyTest :: CtEvidence -> TcTyVar -> TcType -> TcS UnifyTestResult
-- This is the key test for untouchability:
-- See Note [Unification preconditions] in GHC.Tc.Utils.Unify
-- and Note [Solve by unification] in GHC.Tc.Solver.Interact
unifyTest :: CtEvidence -> TyVar -> Xi -> TcS UnifyTestResult
unifyTest CtEvidence
ev TyVar
tv1 Xi
rhs
  | Bool -> Bool
not (CtEvidence -> Bool
isGiven CtEvidence
ev)  -- See Note [Do not unify Givens]
  , MetaTv { mtv_tclvl :: TcTyVarDetails -> TcLevel
mtv_tclvl = TcLevel
tv_lvl, mtv_info :: TcTyVarDetails -> MetaInfo
mtv_info = MetaInfo
info } <- TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv1
  , MetaInfo -> Xi -> Bool
canSolveByUnification MetaInfo
info Xi
rhs
  = do { TcLevel
ambient_lvl  <- TcS TcLevel
getTcLevel
       ; TcLevel
given_eq_lvl <- TcS TcLevel
getInnermostGivenEqLevel

       ; if | TcLevel
tv_lvl TcLevel -> TcLevel -> Bool
`sameDepthAs` TcLevel
ambient_lvl
            -> UnifyTestResult -> TcS UnifyTestResult
forall (m :: * -> *) a. Monad m => a -> m a
return UnifyTestResult
UnifySameLevel

            | TcLevel
tv_lvl TcLevel -> TcLevel -> Bool
`deeperThanOrSame` TcLevel
given_eq_lvl   -- No intervening given equalities
            , (TyVar -> Bool) -> [TyVar] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all (TcLevel -> TyVar -> Bool
does_not_escape TcLevel
tv_lvl) [TyVar]
free_skols  -- No skolem escapes
            -> UnifyTestResult -> TcS UnifyTestResult
forall (m :: * -> *) a. Monad m => a -> m a
return ([TyVar] -> TcLevel -> UnifyTestResult
UnifyOuterLevel [TyVar]
free_metas TcLevel
tv_lvl)

            | Bool
otherwise
            -> UnifyTestResult -> TcS UnifyTestResult
forall (m :: * -> *) a. Monad m => a -> m a
return UnifyTestResult
NoUnify }
  | Bool
otherwise
  = UnifyTestResult -> TcS UnifyTestResult
forall (m :: * -> *) a. Monad m => a -> m a
return UnifyTestResult
NoUnify
  where
     ([TyVar]
free_metas, [TyVar]
free_skols) = (TyVar -> Bool) -> [TyVar] -> ([TyVar], [TyVar])
forall a. (a -> Bool) -> [a] -> ([a], [a])
partition TyVar -> Bool
isPromotableMetaTyVar ([TyVar] -> ([TyVar], [TyVar])) -> [TyVar] -> ([TyVar], [TyVar])
forall a b. (a -> b) -> a -> b
$
                                VarSet -> [TyVar]
forall elt. UniqSet elt -> [elt]
nonDetEltsUniqSet               (VarSet -> [TyVar]) -> VarSet -> [TyVar]
forall a b. (a -> b) -> a -> b
$
                                Xi -> VarSet
tyCoVarsOfType Xi
rhs

     does_not_escape :: TcLevel -> TyVar -> Bool
does_not_escape TcLevel
tv_lvl TyVar
fv
       | TyVar -> Bool
isTyVar TyVar
fv = TcLevel
tv_lvl TcLevel -> TcLevel -> Bool
`deeperThanOrSame` TyVar -> TcLevel
tcTyVarLevel TyVar
fv
       | Bool
otherwise  = Bool
True
       -- Coercion variables are not an escape risk
       -- If an implication binds a coercion variable, it'll have equalities,
       -- so the "intervening given equalities" test above will catch it
       -- Coercion holes get filled with coercions, so again no problem.

{- Note [Do not unify Givens]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this GADT match
   data T a where
      T1 :: T Int
      ...

   f x = case x of
           T1 -> True
           ...

So we get f :: T alpha[1] -> beta[1]
          x :: T alpha[1]
and from the T1 branch we get the implication
   forall[2] (alpha[1] ~ Int) => beta[1] ~ Bool

Now, clearly we don't want to unify alpha:=Int!  Yet at the moment we
process [G] alpha[1] ~ Int, we don't have any given-equalities in the
inert set, and hence there are no given equalities to make alpha untouchable.

(NB: if it were alpha[2] ~ Int, this argument wouldn't hold.  But that
almost never happens, and will never happen at all if we cure #18929.)

Simple solution: never unify in Givens!
-}

-- The RHS here is either not CanEqLHS, or it's one that we
-- want to rewrite the LHS to (as per e.g. swapOverTyVars)
canEqCanLHSFinish :: CtEvidence
                  -> EqRel -> SwapFlag
                  -> CanEqLHS              -- lhs (or, if swapped, rhs)
                  -> TcType          -- rhs, pretty rhs
                  -> TcS (StopOrContinue Ct)
canEqCanLHSFinish :: CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> Xi -> TcS (StopOrContinue Ct)
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Xi
rhs
-- RHS is fully rewritten, but with type synonyms
-- preserved as much as possible
-- guaranteed that tyVarKind lhs == typeKind rhs, for (TyEq:K)
-- (TyEq:N) is checked in can_eq_nc', and (TyEq:TV) is handled in canEqTyVarHomo

  = do { DynFlags
dflags <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; CtEvidence
new_ev <- CtEvidence
-> SwapFlag
-> Xi
-> Xi
-> TcCoercion
-> TcCoercion
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped Xi
lhs_ty Xi
rhs TcCoercion
rewrite_co1 TcCoercion
rewrite_co2

     -- Must do the occurs check even on tyvar/tyvar
     -- equalities, in case have  x ~ (y :: ..x...)
     -- #12593
     -- guarantees (TyEq:OC), (TyEq:F), and (TyEq:H)
    -- this next line checks also for coercion holes (TyEq:H); see
    -- Note [Equalities with incompatible kinds]
       ; case DynFlags -> EqRel -> CanEqLHS -> Xi -> CanEqOK
canEqOK DynFlags
dflags EqRel
eq_rel CanEqLHS
lhs Xi
rhs of
           CanEqOK
CanEqOK ->
             do { String -> SDoc -> TcS ()
traceTcS String
"canEqOK" (CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs SDoc -> SDoc -> SDoc
$$ Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
rhs)
                ; Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (CEqCan :: CtEvidence -> CanEqLHS -> Xi -> EqRel -> Ct
CEqCan { cc_ev :: CtEvidence
cc_ev = CtEvidence
new_ev, cc_lhs :: CanEqLHS
cc_lhs = CanEqLHS
lhs
                                       , cc_rhs :: Xi
cc_rhs = Xi
rhs, cc_eq_rel :: EqRel
cc_eq_rel = EqRel
eq_rel }) }
       -- it is possible that cc_rhs mentions the LHS if the LHS is a type
       -- family. This will cause later rewriting to potentially loop, but
       -- that will be caught by the depth counter. The other option is an
       -- occurs-check for a function application, which seems awkward.

           CanEqNotOK CtIrredStatus
status
                -- See Note [Type variable cycles in Givens]
             | CtIrredStatus
OtherCIS <- CtIrredStatus
status
             , CtFlavour
Given <- CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev
             , TyVarLHS TyVar
lhs_tv <- CanEqLHS
lhs
             , Bool -> Bool
not (TyVar -> Bool
isCycleBreakerTyVar TyVar
lhs_tv) -- See Detail (7) of Note
             , EqRel
NomEq <- EqRel
eq_rel
             -> do { String -> SDoc -> TcS ()
traceTcS String
"canEqCanLHSFinish breaking a cycle" (CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs SDoc -> SDoc -> SDoc
$$ Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
rhs)
                   ; Xi
new_rhs <- CtLoc -> Xi -> TcS Xi
breakTyVarCycle (CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev) Xi
rhs
                   ; String -> SDoc -> TcS ()
traceTcS String
"new RHS:" (Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
new_rhs)
                   ; let new_pred :: Xi
new_pred   = Xi -> Xi -> Xi
mkPrimEqPred (TyVar -> Xi
mkTyVarTy TyVar
lhs_tv) Xi
new_rhs
                         new_new_ev :: CtEvidence
new_new_ev = CtEvidence
new_ev { ctev_pred :: Xi
ctev_pred = Xi
new_pred }
                           -- See Detail (6) of Note [Type variable cycles in Givens]

                   ; if Bool -> EqRel -> (EqRel -> TyVar -> Bool) -> Xi -> Bool
anyRewritableTyVar Bool
True EqRel
NomEq (\ EqRel
_ TyVar
tv -> TyVar
tv TyVar -> TyVar -> Bool
forall a. Eq a => a -> a -> Bool
== TyVar
lhs_tv) Xi
new_rhs
                     then do { String -> SDoc -> TcS ()
traceTcS String
"Note [Type variable cycles in Givens] Detail (1)"
                                        (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
new_new_ev)
                             ; Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredStatus -> CtEvidence -> Ct
mkIrredCt CtIrredStatus
status CtEvidence
new_ev) }
                     else Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (CEqCan :: CtEvidence -> CanEqLHS -> Xi -> EqRel -> Ct
CEqCan { cc_ev :: CtEvidence
cc_ev = CtEvidence
new_new_ev, cc_lhs :: CanEqLHS
cc_lhs = CanEqLHS
lhs
                                               , cc_rhs :: Xi
cc_rhs = Xi
new_rhs, cc_eq_rel :: EqRel
cc_eq_rel = EqRel
eq_rel }) }

               -- We must not use it for further rewriting!
             | Bool
otherwise
             -> do { String -> SDoc -> TcS ()
traceTcS String
"canEqCanLHSFinish can't make a canonical" (CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs SDoc -> SDoc -> SDoc
$$ Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
rhs)
                   ; Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS (StopOrContinue a)
continueWith (CtIrredStatus -> CtEvidence -> Ct
mkIrredCt CtIrredStatus
status CtEvidence
new_ev) } }
  where
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

    lhs_ty :: Xi
lhs_ty = CanEqLHS -> Xi
canEqLHSType CanEqLHS
lhs

    rewrite_co1 :: TcCoercion
rewrite_co1  = Role -> Xi -> TcCoercion
mkTcReflCo Role
role Xi
lhs_ty
    rewrite_co2 :: TcCoercion
rewrite_co2  = Role -> Xi -> TcCoercion
mkTcReflCo Role
role Xi
rhs

-- | Solve a reflexive equality constraint
canEqReflexive :: CtEvidence    -- ty ~ ty
               -> EqRel
               -> TcType        -- ty
               -> TcS (StopOrContinue Ct)   -- always Stop
canEqReflexive :: CtEvidence -> EqRel -> Xi -> TcS (StopOrContinue Ct)
canEqReflexive CtEvidence
ev EqRel
eq_rel Xi
ty
  = do { CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev (TcCoercion -> EvTerm
evCoercion (TcCoercion -> EvTerm) -> TcCoercion -> EvTerm
forall a b. (a -> b) -> a -> b
$
                               Role -> Xi -> TcCoercion
mkTcReflCo (EqRel -> Role
eqRelRole EqRel
eq_rel) Xi
ty)
       ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Solved by reflexivity" }

rewriteCastedEquality :: CtEvidence     -- :: lhs ~ (rhs |> mco), or (rhs |> mco) ~ lhs
                      -> EqRel -> SwapFlag
                      -> TcType         -- lhs
                      -> TcType         -- rhs
                      -> MCoercion      -- mco
                      -> TcS CtEvidence -- :: (lhs |> sym mco) ~ rhs
                                        -- result is independent of SwapFlag
rewriteCastedEquality :: CtEvidence
-> EqRel -> SwapFlag -> Xi -> Xi -> MCoercion -> TcS CtEvidence
rewriteCastedEquality CtEvidence
ev EqRel
eq_rel SwapFlag
swapped Xi
lhs Xi
rhs MCoercion
mco
  = CtEvidence
-> SwapFlag
-> Xi
-> Xi
-> TcCoercion
-> TcCoercion
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped Xi
new_lhs Xi
new_rhs TcCoercion
lhs_co TcCoercion
rhs_co
  where
    new_lhs :: Xi
new_lhs = Xi
lhs Xi -> MCoercion -> Xi
`mkCastTyMCo` MCoercion
sym_mco
    lhs_co :: TcCoercion
lhs_co  = Role -> Xi -> MCoercion -> TcCoercion
mkTcGReflLeftMCo Role
role Xi
lhs MCoercion
sym_mco

    new_rhs :: Xi
new_rhs = Xi
rhs
    rhs_co :: TcCoercion
rhs_co  = Role -> Xi -> MCoercion -> TcCoercion
mkTcGReflRightMCo Role
role Xi
rhs MCoercion
mco

    sym_mco :: MCoercion
sym_mco = MCoercion -> MCoercion
mkTcSymMCo MCoercion
mco
    role :: Role
role    = EqRel -> Role
eqRelRole EqRel
eq_rel

---------------------------------------------
-- | Result of checking whether a RHS is suitable for pairing
-- with a CanEqLHS in a CEqCan.
data CanEqOK
  = CanEqOK                   -- RHS is good
  | CanEqNotOK CtIrredStatus  -- don't proceed; explains why

instance Outputable CanEqOK where
  ppr :: CanEqOK -> SDoc
ppr CanEqOK
CanEqOK             = String -> SDoc
text String
"CanEqOK"
  ppr (CanEqNotOK CtIrredStatus
status) = String -> SDoc
text String
"CanEqNotOK" SDoc -> SDoc -> SDoc
<+> CtIrredStatus -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtIrredStatus
status

-- | This function establishes most of the invariants needed to make
-- a CEqCan.
--
--   TyEq:OC: Checked here.
--   TyEq:F:  Checked here.
--   TyEq:K:  assumed; ASSERTed here (that is, kind(lhs) = kind(rhs))
--   TyEq:N:  assumed; ASSERTed here (if eq_rel is R, rhs is not a newtype)
--   TyEq:TV: not checked (this is hard to check)
--   TyEq:H:  Checked here.
canEqOK :: DynFlags -> EqRel -> CanEqLHS -> Xi -> CanEqOK
canEqOK :: DynFlags -> EqRel -> CanEqLHS -> Xi -> CanEqOK
canEqOK DynFlags
dflags EqRel
eq_rel CanEqLHS
lhs Xi
rhs
  = ASSERT( good_rhs )
    case DynFlags -> AreTypeFamiliesOK -> CanEqLHS -> Xi -> CheckTyEqResult
checkTypeEq DynFlags
dflags AreTypeFamiliesOK
YesTypeFamilies CanEqLHS
lhs Xi
rhs of
      CheckTyEqResult
CTE_OK  -> CanEqOK
CanEqOK
      CheckTyEqResult
CTE_Bad -> CtIrredStatus -> CanEqOK
CanEqNotOK CtIrredStatus
OtherCIS
                 -- Violation of TyEq:F

      CheckTyEqResult
CTE_HoleBlocker -> CtIrredStatus -> CanEqOK
CanEqNotOK (HoleSet -> CtIrredStatus
BlockedCIS HoleSet
holes)
        where holes :: HoleSet
holes = Xi -> HoleSet
coercionHolesOfType Xi
rhs
                 -- This is the case detailed in
                 -- Note [Equalities with incompatible kinds]
                 -- Violation of TyEq:H

                 -- These are both a violation of TyEq:OC, but we
                 -- want to differentiate for better production of
                 -- error messages
      CheckTyEqResult
CTE_Occurs | TyVarLHS TyVar
tv <- CanEqLHS
lhs
                  , EqRel -> TyVar -> Xi -> Bool
isInsolubleOccursCheck EqRel
eq_rel TyVar
tv Xi
rhs -> CtIrredStatus -> CanEqOK
CanEqNotOK CtIrredStatus
InsolubleCIS
                 -- If we have a ~ [a], it is not canonical, and in particular
                 -- we don't want to rewrite existing inerts with it, otherwise
                 -- we'd risk divergence in the constraint solver

                 -- NB: no occCheckExpand here; see Note [Rewriting synonyms]
                 -- in GHC.Tc.Solver.Rewrite

                  | Bool
otherwise                            -> CtIrredStatus -> CanEqOK
CanEqNotOK CtIrredStatus
OtherCIS
                 -- A representational equality with an occurs-check problem isn't
                 -- insoluble! For example:
                 --   a ~R b a
                 -- We might learn that b is the newtype Id.
                 -- But, the occurs-check certainly prevents the equality from being
                 -- canonical, and we might loop if we were to use it in rewriting.

                 -- This case also include type family occurs-check errors, which
                 -- are not generally insoluble

  where
    good_rhs :: Bool
good_rhs    = Bool
kinds_match Bool -> Bool -> Bool
&& Bool -> Bool
not Bool
bad_newtype

    lhs_kind :: Xi
lhs_kind    = CanEqLHS -> Xi
canEqLHSKind CanEqLHS
lhs
    rhs_kind :: Xi
rhs_kind    = HasDebugCallStack => Xi -> Xi
Xi -> Xi
tcTypeKind Xi
rhs

    kinds_match :: Bool
kinds_match = Xi
lhs_kind HasDebugCallStack => Xi -> Xi -> Bool
Xi -> Xi -> Bool
`tcEqType` Xi
rhs_kind

    bad_newtype :: Bool
bad_newtype | EqRel
ReprEq <- EqRel
eq_rel
                , Just TyCon
tc <- Xi -> Maybe TyCon
tyConAppTyCon_maybe Xi
rhs
                = TyCon -> Bool
isNewTyCon TyCon
tc
                | Bool
otherwise
                = Bool
False

{- Note [Equalities with incompatible kinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
What do we do when we have an equality

  (tv :: k1) ~ (rhs :: k2)

where k1 and k2 differ? Easy: we create a coercion that relates k1 and
k2 and use this to cast. To wit, from

  [X] (tv :: k1) ~ (rhs :: k2)

(where [X] is [G], [W], or [D]), we go to

  [noDerived X] co :: k2 ~ k1
  [X]           (tv :: k1) ~ ((rhs |> co) :: k1)

where

  noDerived G = G
  noDerived _ = W

For reasons described in Wrinkle (2) below, we want the [X] constraint to be "blocked";
that is, it should be put aside, and not used to rewrite any other constraint,
until the kind-equality on which it depends (namely 'co' above) is solved.
To achieve this
* The [X] constraint is a CIrredCan
* With a cc_status of BlockedCIS bchs
* Where 'bchs' is the set of "blocking coercion holes".  The blocking coercion
  holes are the free coercion holes of [X]'s type
* When all the blocking coercion holes in the CIrredCan are filled (solved),
  we convert [X] to a CNonCanonical and put it in the work list.
All this is described in more detail in Wrinkle (2).

Wrinkles:

 (1) The noDerived step is because Derived equalities have no evidence.
     And yet we absolutely need evidence to be able to proceed here.
     Given evidence will use the KindCo coercion; Wanted evidence will
     be a coercion hole. Even a Derived hetero equality begets a Wanted
     kind equality.

 (2) Though it would be sound to do so, we must not mark the rewritten Wanted
       [W] (tv :: k1) ~ ((rhs |> co) :: k1)
     as canonical in the inert set. In particular, we must not unify tv.
     If we did, the Wanted becomes a Given (effectively), and then can
     rewrite other Wanteds. But that's bad: See Note [Wanteds do not rewrite Wanteds]
     in GHC.Tc.Types.Constraint. The problem is about poor error messages. See #11198 for
     tales of destruction.

     So, we have an invariant on CEqCan (TyEq:H) that the RHS does not have
     any coercion holes. This is checked in checkTypeEq. Any equalities that
     have such an RHS are turned into CIrredCans with a BlockedCIS status. We also
     must be sure to kick out any such CIrredCan constraints that mention coercion holes
     when those holes get filled in, so that the unification step can now proceed.

     The kicking out is done in kickOutAfterFillingCoercionHole.

     However, we must be careful: we kick out only when no coercion holes are
     left. The holes in the type are stored in the BlockedCIS CtIrredStatus.
     The extra check that there are no more remaining holes avoids
     needless work when rewriting evidence (which fills coercion holes) and
     aids efficiency.

     Moreover, kicking out when there are remaining unfilled holes can
     cause a loop in the solver in this case:
          [W] w1 :: (ty1 :: F a) ~ (ty2 :: s)
     After canonicalisation, we discover that this equality is heterogeneous.
     So we emit
          [W] co_abc :: F a ~ s
     and preserve the original as
          [W] w2 :: (ty1 |> co_abc) ~ ty2    (blocked on co_abc)
     Then, co_abc comes becomes the work item. It gets swapped in
     canEqCanLHS2 and then back again in canEqTyVarFunEq. We thus get
     co_abc := sym co_abd, and then co_abd := sym co_abe, with
          [W] co_abe :: F a ~ s
     This process has filled in co_abc. Suppose w2 were kicked out.
     When it gets processed,
     would get this whole chain going again. The solution is to
     kick out a blocked constraint only when the result of filling
     in the blocking coercion involves no further blocking coercions.
     Alternatively, we could be careful not to do unnecessary swaps during
     canonicalisation, but that seems hard to do, in general.

 (3) Suppose we have [W] (a :: k1) ~ (rhs :: k2). We duly follow the
     algorithm detailed here, producing [W] co :: k2 ~ k1, and adding
     [W] (a :: k1) ~ ((rhs |> co) :: k1) to the irreducibles. Some time
     later, we solve co, and fill in co's coercion hole. This kicks out
     the irreducible as described in (2).
     But now, during canonicalization, we see the cast
     and remove it, in canEqCast. By the time we get into canEqCanLHS, the equality
     is heterogeneous again, and the process repeats.

     To avoid this, we don't strip casts off a type if the other type
     in the equality is a CanEqLHS (the scenario above can happen with a
     type family, too. testcase: typecheck/should_compile/T13822).
     And this is an improvement regardless:
     because tyvars can, generally, unify with casted types, there's no
     reason to go through the work of stripping off the cast when the
     cast appears opposite a tyvar. This is implemented in the cast case
     of can_eq_nc'.

 (4) Reporting an error for a constraint that is blocked with status BlockedCIS
     is hard: what would we say to users? And we don't
     really need to report, because if a constraint is blocked, then
     there is unsolved wanted blocking it; that unsolved wanted will
     be reported. We thus push such errors to the bottom of the queue
     in the error-reporting code; they should never be printed.

     (4a) It would seem possible to do this filtering just based on the
          presence of a blocking coercion hole. However, this is no good,
          as it suppresses e.g. no-instance-found errors. We thus record
          a CtIrredStatus in CIrredCan and filter based on this status.
          This happened in T14584. An alternative approach is to expressly
          look for *equalities* with blocking coercion holes, but actually
          recording the blockage in a status field seems nicer.

     (4b) The error message might be printed with -fdefer-type-errors,
          so it still must exist. This is the only reason why there is
          a message at all. Otherwise, we could simply do nothing.

Historical note:

We used to do this via emitting a Derived kind equality and then parking
the heterogeneous equality as irreducible. But this new approach is much
more direct. And it doesn't produce duplicate Deriveds (as the old one did).

Note [Type synonyms and canonicalization]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We treat type synonym applications as xi types, that is, they do not
count as type function applications.  However, we do need to be a bit
careful with type synonyms: like type functions they may not be
generative or injective.  However, unlike type functions, they are
parametric, so there is no problem in expanding them whenever we see
them, since we do not need to know anything about their arguments in
order to expand them; this is what justifies not having to treat them
as specially as type function applications.  The thing that causes
some subtleties is that we prefer to leave type synonym applications
*unexpanded* whenever possible, in order to generate better error
messages.

If we encounter an equality constraint with type synonym applications
on both sides, or a type synonym application on one side and some sort
of type application on the other, we simply must expand out the type
synonyms in order to continue decomposing the equality constraint into
primitive equality constraints.  For example, suppose we have

  type F a = [Int]

and we encounter the equality

  F a ~ [b]

In order to continue we must expand F a into [Int], giving us the
equality

  [Int] ~ [b]

which we can then decompose into the more primitive equality
constraint

  Int ~ b.

However, if we encounter an equality constraint with a type synonym
application on one side and a variable on the other side, we should
NOT (necessarily) expand the type synonym, since for the purpose of
good error messages we want to leave type synonyms unexpanded as much
as possible.  Hence the ps_xi1, ps_xi2 argument passed to canEqCanLHS.

Note [Type variable cycles in Givens]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this situation (from indexed-types/should_compile/GivenLoop):

  instance C (Maybe b)
  [G] a ~ Maybe (F a)
  [W] C a

In order to solve the Wanted, we must use the Given to rewrite `a` to
Maybe (F a). But note that the Given has an occurs-check failure, and
so we can't straightforwardly add the Given to the inert set.

The key idea is to replace the (F a) in the RHS of the Given with a
fresh variable, which we'll call a CycleBreakerTv, or cbv. Then, emit
a new Given to connect cbv with F a. So our situation becomes

  instance C (Maybe b)
  [G] a ~ Maybe cbv
  [G] F a ~ cbv
  [W] C a

Note the orientation of the second Given. The type family ends up
on the left; see commentary on canEqTyVarFunEq, which decides how to
orient such cases. No special treatment for CycleBreakerTvs is
necessary. This scenario is now easily soluble, by using the first
Given to rewrite the Wanted, which can now be solved.

(The first Given actually also rewrites the second one. This causes
no trouble.)

More generally, we detect this scenario by the following characteristics:
 - a Given CEqCan constraint
 - with a tyvar on its LHS
 - with a soluble occurs-check failure
 - and a nominal equality

Having identified the scenario, we wish to replace all type family
applications on the RHS with fresh metavariables (with MetaInfo
CycleBreakerTv). This is done in breakTyVarCycle. These metavariables are
untouchable, but we also emit Givens relating the fresh variables to the type
family applications they replace.

Of course, we don't want our fresh variables leaking into e.g. error messages.
So we fill in the metavariables with their original type family applications
after we're done running the solver (in nestImplicTcS and runTcSWithEvBinds).
This is done by restoreTyVarCycles, which uses the inert_cycle_breakers field in
InertSet, which contains the pairings invented in breakTyVarCycle.

That is:

We transform
  [G] g : a ~ ...(F a)...
to
  [G] (Refl a) : F a ~ cbv      -- CEqCan
  [G] g        : a ~ ...cbv...  -- CEqCan

Note that
* `cbv` is a fresh cycle breaker variable.
* `cbv` is a is a meta-tyvar, but it is completely untouchable.
* We track the cycle-breaker variables in inert_cycle_breakers in InertSet
* We eventually fill in the cycle-breakers, with `cbv := F a`.
  No one else fills in cycle-breakers!
* In inert_cycle_breakers, we remember the (cbv, F a) pair; that is, we
  remember the /original/ type.  The [G] F a ~ cbv constraint may be rewritten
  by other givens (eg if we have another [G] a ~ (b,c), but at the end we
  still fill in with cbv := F a
* This fill-in is done when solving is complete, by restoreTyVarCycles
  in nestImplicTcS and runTcSWithEvBinds.
* The evidence for the new `F a ~ cbv` constraint is Refl, because we know this fill-in is
  ultimately going to happen.

There are drawbacks of this approach:

 1. We apply this trick only for Givens, never for Wanted or Derived.
    It wouldn't make sense for Wanted, because Wanted never rewrite.
    But it's conceivable that a Derived would benefit from this all.
    I doubt it would ever happen, though, so I'm holding off.

 2. We don't use this trick for representational equalities, as there
    is no concrete use case where it is helpful (unlike for nominal
    equalities). Furthermore, because function applications can be
    CanEqLHSs, but newtype applications cannot, the disparities between
    the cases are enough that it would be effortful to expand the idea
    to representational equalities. A quick attempt, with

      data family N a b

      f :: (Coercible a (N a b), Coercible (N a b) b) => a -> b
      f = coerce

    failed with "Could not match 'b' with 'b'." Further work is held off
    until when we have a concrete incentive to explore this dark corner.

Details:

 (1) We don't look under foralls, at all, when substituting away type family
     applications, because doing so can never be fruitful. Recall that we
     are in a case like [G] a ~ forall b. ... a ....   Until we have a type
     family that can pull the body out from a forall, this will always be
     insoluble. Note also that the forall cannot be in an argument to a
     type family, or that outer type family application would already have
     been substituted away.

     However, we still must check to make sure that breakTyVarCycle actually
     succeeds in getting rid of all occurrences of the offending variable.
     If one is hidden under a forall, this won't be true. So we perform
     an additional check after performing the substitution.

     Skipping this check causes typecheck/should_fail/GivenForallLoop to loop.

 (2) Our goal here is to avoid loops in rewriting. We can thus skip looking
     in coercions, as we don't rewrite in coercions.
     (There is no worry about unifying a meta-variable here: this Note is
      only about Givens.)

 (3) As we're substituting, we can build ill-kinded
     types. For example, if we have Proxy (F a) b, where (b :: F a), then
     replacing this with Proxy cbv b is ill-kinded. However, we will later
     set cbv := F a, and so the zonked type will be well-kinded again.
     The temporary ill-kinded type hurts no one, and avoiding this would
     be quite painfully difficult.

     Specifically, this detail does not contravene the Purely Kinded Type Invariant
     (Note [The Purely Kinded Type Invariant (PKTI)] in GHC.Tc.Gen.HsType).
     The PKTI says that we can call typeKind on any type, without failure.
     It would be violated if we, say, replaced a kind (a -> b) with a kind c,
     because an arrow kind might be consulted in piResultTys. Here, we are
     replacing one opaque type like (F a b c) with another, cbv (opaque in
     that we never assume anything about its structure, like that it has a
     result type or a RuntimeRep argument).

 (4) The evidence for the produced Givens is all just reflexive, because
     we will eventually set the cycle-breaker variable to be the type family,
     and then, after the zonk, all will be well.

 (5) The approach here is inefficient. For instance, we could choose to
     affect only type family applications that mention the offending variable:
     in a ~ (F b, G a), we need to replace only G a, not F b. Furthermore,
     we could try to detect cases like a ~ (F a, F a) and use the same
     tyvar to replace F a. (Cf.
     Note [Flattening type-family applications when matching instances]
     in GHC.Core.Unify, which
     goes to this extra effort.) There may be other opportunities for
     improvement. However, this is really a very small corner case, always
     tickled by a user-written Given. The investment to craft a clever,
     performant solution seems unworthwhile.

 (6) We often get the predicate associated with a constraint from its
     evidence. We thus must not only make sure the generated CEqCan's
     fields have the updated RHS type, but we must also update the
     evidence itself. As in Detail (4), we don't need to change the
     evidence term (as in e.g. rewriteEqEvidence) because the cycle
     breaker variables are all zonked away by the time we examine the
     evidence. That is, we must set the ctev_pred of the ctEvidence.
     This is implemented in canEqCanLHSFinish, with a reference to
     this detail.

 (7) We don't wish to apply this magic to CycleBreakerTvs themselves.
     Consider this, from typecheck/should_compile/ContextStack2:

       type instance TF (a, b) = (TF a, TF b)
       t :: (a ~ TF (a, Int)) => ...

       [G] a ~ TF (a, Int)

     The RHS reduces, so we get

       [G] a ~ (TF a, TF Int)

     We then break cycles, to get

       [G] g1 :: a ~ (cbv1, cbv2)
       [G] g2 :: TF a ~ cbv1
       [G] g3 :: TF Int ~ cbv2

     g1 gets added to the inert set, as written. But then g2 becomes
     the work item. g1 rewrites g2 to become

       [G] TF (cbv1, cbv2) ~ cbv1

     which then uses the type instance to become

       [G] (TF cbv1, TF cbv2) ~ cbv1

     which looks remarkably like the Given we started with. If left
     unchecked, this will end up breaking cycles again, looping ad
     infinitum (and resulting in a context-stack reduction error,
     not an outright loop). The solution is easy: don't break cycles
     if the var is already a CycleBreakerTv. Instead, we mark this
     final Given as a CIrredCan with an OtherCIS status (it's not
     insoluble).

     NB: When filling in CycleBreakerTvs, we fill them in with what
     they originally stood for (e.g. cbv1 := TF a, cbv2 := TF Int),
     not what may be in a rewritten constraint.

     Not breaking cycles further (which would mean changing TF cbv1 to cbv3
     and TF cbv2 to cbv4) makes sense, because we only want to break cycles
     for user-written loopy Givens, and a CycleBreakerTv certainly isn't
     user-written.

NB: This same situation (an equality like b ~ Maybe (F b)) can arise with
Wanteds, but we have no concrete case incentivising special treatment. It
would just be a CIrredCan.

-}

{-
************************************************************************
*                                                                      *
                  Evidence transformation
*                                                                      *
************************************************************************
-}

data StopOrContinue a
  = ContinueWith a    -- The constraint was not solved, although it may have
                      --   been rewritten

  | Stop CtEvidence   -- The (rewritten) constraint was solved
         SDoc         -- Tells how it was solved
                      -- Any new sub-goals have been put on the work list
  deriving (a -> StopOrContinue b -> StopOrContinue a
(a -> b) -> StopOrContinue a -> StopOrContinue b
(forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b)
-> (forall a b. a -> StopOrContinue b -> StopOrContinue a)
-> Functor StopOrContinue
forall a b. a -> StopOrContinue b -> StopOrContinue a
forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b
forall (f :: * -> *).
(forall a b. (a -> b) -> f a -> f b)
-> (forall a b. a -> f b -> f a) -> Functor f
<$ :: a -> StopOrContinue b -> StopOrContinue a
$c<$ :: forall a b. a -> StopOrContinue b -> StopOrContinue a
fmap :: (a -> b) -> StopOrContinue a -> StopOrContinue b
$cfmap :: forall a b. (a -> b) -> StopOrContinue a -> StopOrContinue b
Functor)

instance Outputable a => Outputable (StopOrContinue a) where
  ppr :: StopOrContinue a -> SDoc
ppr (Stop CtEvidence
ev SDoc
s)      = String -> SDoc
text String
"Stop" SDoc -> SDoc -> SDoc
<> SDoc -> SDoc
parens SDoc
s SDoc -> SDoc -> SDoc
<+> CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
  ppr (ContinueWith a
w) = String -> SDoc
text String
"ContinueWith" SDoc -> SDoc -> SDoc
<+> a -> SDoc
forall a. Outputable a => a -> SDoc
ppr a
w

continueWith :: a -> TcS (StopOrContinue a)
continueWith :: a -> TcS (StopOrContinue a)
continueWith = StopOrContinue a -> TcS (StopOrContinue a)
forall (m :: * -> *) a. Monad m => a -> m a
return (StopOrContinue a -> TcS (StopOrContinue a))
-> (a -> StopOrContinue a) -> a -> TcS (StopOrContinue a)
forall b c a. (b -> c) -> (a -> b) -> a -> c
. a -> StopOrContinue a
forall a. a -> StopOrContinue a
ContinueWith

stopWith :: CtEvidence -> String -> TcS (StopOrContinue a)
stopWith :: CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
s = StopOrContinue a -> TcS (StopOrContinue a)
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> SDoc -> StopOrContinue a
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev (String -> SDoc
text String
s))

andWhenContinue :: TcS (StopOrContinue a)
                -> (a -> TcS (StopOrContinue b))
                -> TcS (StopOrContinue b)
andWhenContinue :: TcS (StopOrContinue a)
-> (a -> TcS (StopOrContinue b)) -> TcS (StopOrContinue b)
andWhenContinue TcS (StopOrContinue a)
tcs1 a -> TcS (StopOrContinue b)
tcs2
  = do { StopOrContinue a
r <- TcS (StopOrContinue a)
tcs1
       ; case StopOrContinue a
r of
           Stop CtEvidence
ev SDoc
s       -> StopOrContinue b -> TcS (StopOrContinue b)
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> SDoc -> StopOrContinue b
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev SDoc
s)
           ContinueWith a
ct -> a -> TcS (StopOrContinue b)
tcs2 a
ct }
infixr 0 `andWhenContinue`    -- allow chaining with ($)

rewriteEvidence :: CtEvidence   -- old evidence
                -> TcPredType   -- new predicate
                -> TcCoercion   -- Of type :: new predicate ~ <type of old evidence>
                -> TcS (StopOrContinue CtEvidence)
-- Returns Just new_ev iff either (i)  'co' is reflexivity
--                             or (ii) 'co' is not reflexivity, and 'new_pred' not cached
-- In either case, there is nothing new to do with new_ev
{-
     rewriteEvidence old_ev new_pred co
Main purpose: create new evidence for new_pred;
              unless new_pred is cached already
* Returns a new_ev : new_pred, with same wanted/given/derived flag as old_ev
* If old_ev was wanted, create a binding for old_ev, in terms of new_ev
* If old_ev was given, AND not cached, create a binding for new_ev, in terms of old_ev
* Returns Nothing if new_ev is already cached

        Old evidence    New predicate is               Return new evidence
        flavour                                        of same flavor
        -------------------------------------------------------------------
        Wanted          Already solved or in inert     Nothing
        or Derived      Not                            Just new_evidence

        Given           Already in inert               Nothing
                        Not                            Just new_evidence

Note [Rewriting with Refl]
~~~~~~~~~~~~~~~~~~~~~~~~~~
If the coercion is just reflexivity then you may re-use the same
variable.  But be careful!  Although the coercion is Refl, new_pred
may reflect the result of unification alpha := ty, so new_pred might
not _look_ the same as old_pred, and it's vital to proceed from now on
using new_pred.

The rewriter preserves type synonyms, so they should appear in new_pred
as well as in old_pred; that is important for good error messages.
 -}


rewriteEvidence :: CtEvidence -> Xi -> TcCoercion -> TcS (StopOrContinue CtEvidence)
rewriteEvidence old_ev :: CtEvidence
old_ev@(CtDerived {}) Xi
new_pred TcCoercion
_co
  = -- If derived, don't even look at the coercion.
    -- This is very important, DO NOT re-order the equations for
    -- rewriteEvidence to put the isTcReflCo test first!
    -- Why?  Because for *Derived* constraints, c, the coercion, which
    -- was produced by rewriting, may contain suspended calls to
    -- (ctEvExpr c), which fails for Derived constraints.
    -- (Getting this wrong caused #7384.)
    CtEvidence -> TcS (StopOrContinue CtEvidence)
forall a. a -> TcS (StopOrContinue a)
continueWith (CtEvidence
old_ev { ctev_pred :: Xi
ctev_pred = Xi
new_pred })

rewriteEvidence CtEvidence
old_ev Xi
new_pred TcCoercion
co
  | TcCoercion -> Bool
isTcReflCo TcCoercion
co -- See Note [Rewriting with Refl]
  = CtEvidence -> TcS (StopOrContinue CtEvidence)
forall a. a -> TcS (StopOrContinue a)
continueWith (CtEvidence
old_ev { ctev_pred :: Xi
ctev_pred = Xi
new_pred })

rewriteEvidence ev :: CtEvidence
ev@(CtGiven { ctev_evar :: CtEvidence -> TyVar
ctev_evar = TyVar
old_evar, ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc }) Xi
new_pred TcCoercion
co
  = do { CtEvidence
new_ev <- CtLoc -> (Xi, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc (Xi
new_pred, EvTerm
new_tm)
       ; CtEvidence -> TcS (StopOrContinue CtEvidence)
forall a. a -> TcS (StopOrContinue a)
continueWith CtEvidence
new_ev }
  where
    -- mkEvCast optimises ReflCo
    new_tm :: EvTerm
new_tm = EvExpr -> TcCoercion -> EvTerm
mkEvCast (TyVar -> EvExpr
evId TyVar
old_evar) (Role -> Role -> TcCoercion -> TcCoercion
tcDowngradeRole Role
Representational
                                                       (CtEvidence -> Role
ctEvRole CtEvidence
ev)
                                                       (TcCoercion -> TcCoercion
mkTcSymCo TcCoercion
co))

rewriteEvidence ev :: CtEvidence
ev@(CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest
                             , ctev_nosh :: CtEvidence -> ShadowInfo
ctev_nosh = ShadowInfo
si
                             , ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc }) Xi
new_pred TcCoercion
co
  = do { MaybeNew
mb_new_ev <- ShadowInfo -> CtLoc -> Xi -> TcS MaybeNew
newWanted_SI ShadowInfo
si CtLoc
loc Xi
new_pred
               -- The "_SI" variant ensures that we make a new Wanted
               -- with the same shadow-info as the existing one
               -- with the same shadow-info as the existing one (#16735)
       ; MASSERT( tcCoercionRole co == ctEvRole ev )
       ; TcEvDest -> EvTerm -> TcS ()
setWantedEvTerm TcEvDest
dest
            (EvExpr -> TcCoercion -> EvTerm
mkEvCast (MaybeNew -> EvExpr
getEvExpr MaybeNew
mb_new_ev)
                      (Role -> Role -> TcCoercion -> TcCoercion
tcDowngradeRole Role
Representational (CtEvidence -> Role
ctEvRole CtEvidence
ev) TcCoercion
co))
       ; case MaybeNew
mb_new_ev of
            Fresh  CtEvidence
new_ev -> CtEvidence -> TcS (StopOrContinue CtEvidence)
forall a. a -> TcS (StopOrContinue a)
continueWith CtEvidence
new_ev
            Cached EvExpr
_      -> CtEvidence -> String -> TcS (StopOrContinue CtEvidence)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Cached wanted" }


rewriteEqEvidence :: CtEvidence         -- Old evidence :: olhs ~ orhs (not swapped)
                                        --              or orhs ~ olhs (swapped)
                  -> SwapFlag
                  -> TcType -> TcType   -- New predicate  nlhs ~ nrhs
                  -> TcCoercion         -- lhs_co, of type :: nlhs ~ olhs
                  -> TcCoercion         -- rhs_co, of type :: nrhs ~ orhs
                  -> TcS CtEvidence     -- Of type nlhs ~ nrhs
-- For (rewriteEqEvidence (Given g olhs orhs) False nlhs nrhs lhs_co rhs_co)
-- we generate
-- If not swapped
--      g1 : nlhs ~ nrhs = lhs_co ; g ; sym rhs_co
-- If 'swapped'
--      g1 : nlhs ~ nrhs = lhs_co ; Sym g ; sym rhs_co
--
-- For (Wanted w) we do the dual thing.
-- New  w1 : nlhs ~ nrhs
-- If not swapped
--      w : olhs ~ orhs = sym lhs_co ; w1 ; rhs_co
-- If swapped
--      w : orhs ~ olhs = sym rhs_co ; sym w1 ; lhs_co
--
-- It's all a form of rewwriteEvidence, specialised for equalities
rewriteEqEvidence :: CtEvidence
-> SwapFlag
-> Xi
-> Xi
-> TcCoercion
-> TcCoercion
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
old_ev SwapFlag
swapped Xi
nlhs Xi
nrhs TcCoercion
lhs_co TcCoercion
rhs_co
  | CtDerived {} <- CtEvidence
old_ev  -- Don't force the evidence for a Derived
  = CtEvidence -> TcS CtEvidence
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence
old_ev { ctev_pred :: Xi
ctev_pred = Xi
new_pred })

  | SwapFlag
NotSwapped <- SwapFlag
swapped
  , TcCoercion -> Bool
isTcReflCo TcCoercion
lhs_co      -- See Note [Rewriting with Refl]
  , TcCoercion -> Bool
isTcReflCo TcCoercion
rhs_co
  = CtEvidence -> TcS CtEvidence
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence
old_ev { ctev_pred :: Xi
ctev_pred = Xi
new_pred })

  | CtGiven { ctev_evar :: CtEvidence -> TyVar
ctev_evar = TyVar
old_evar } <- CtEvidence
old_ev
  = do { let new_tm :: EvTerm
new_tm = TcCoercion -> EvTerm
evCoercion (TcCoercion
lhs_co
                                  TcCoercion -> TcCoercion -> TcCoercion
`mkTcTransCo` SwapFlag -> TcCoercion -> TcCoercion
maybeTcSymCo SwapFlag
swapped (TyVar -> TcCoercion
mkTcCoVarCo TyVar
old_evar)
                                  TcCoercion -> TcCoercion -> TcCoercion
`mkTcTransCo` TcCoercion -> TcCoercion
mkTcSymCo TcCoercion
rhs_co)
       ; CtLoc -> (Xi, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc' (Xi
new_pred, EvTerm
new_tm) }

  | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest, ctev_nosh :: CtEvidence -> ShadowInfo
ctev_nosh = ShadowInfo
si } <- CtEvidence
old_ev
  = do { (CtEvidence
new_ev, TcCoercion
hole_co) <- ShadowInfo
-> CtLoc -> Role -> Xi -> Xi -> TcS (CtEvidence, TcCoercion)
newWantedEq_SI ShadowInfo
si CtLoc
loc'
                                             (CtEvidence -> Role
ctEvRole CtEvidence
old_ev) Xi
nlhs Xi
nrhs
               -- The "_SI" variant ensures that we make a new Wanted
               -- with the same shadow-info as the existing one (#16735)
       ; let co :: TcCoercion
co = SwapFlag -> TcCoercion -> TcCoercion
maybeTcSymCo SwapFlag
swapped (TcCoercion -> TcCoercion) -> TcCoercion -> TcCoercion
forall a b. (a -> b) -> a -> b
$
                  TcCoercion -> TcCoercion
mkSymCo TcCoercion
lhs_co
                  TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion
hole_co
                  TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion
rhs_co
       ; TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest TcCoercion
co
       ; String -> SDoc -> TcS ()
traceTcS String
"rewriteEqEvidence" ([SDoc] -> SDoc
vcat [CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
old_ev, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
nlhs, Xi -> SDoc
forall a. Outputable a => a -> SDoc
ppr Xi
nrhs, TcCoercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcCoercion
co])
       ; CtEvidence -> TcS CtEvidence
forall (m :: * -> *) a. Monad m => a -> m a
return CtEvidence
new_ev }

#if __GLASGOW_HASKELL__ <= 810
  | Bool
otherwise
  = String -> TcS CtEvidence
forall a. String -> a
panic String
"rewriteEvidence"
#endif
  where
    new_pred :: Xi
new_pred = CtEvidence -> Xi -> Xi -> Xi
mkTcEqPredLikeEv CtEvidence
old_ev Xi
nlhs Xi
nrhs

      -- equality is like a type class. Bumping the depth is necessary because
      -- of recursive newtypes, where "reducing" a newtype can actually make
      -- it bigger. See Note [Newtypes can blow the stack].
    loc :: CtLoc
loc      = CtEvidence -> CtLoc
ctEvLoc CtEvidence
old_ev
    loc' :: CtLoc
loc'     = CtLoc -> CtLoc
bumpCtLocDepth CtLoc
loc

{-
************************************************************************
*                                                                      *
              Unification
*                                                                      *
************************************************************************

Note [unifyWanted and unifyDerived]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When decomposing equalities we often create new wanted constraints for
(s ~ t).  But what if s=t?  Then it'd be faster to return Refl right away.
Similar remarks apply for Derived.

Rather than making an equality test (which traverses the structure of the
type, perhaps fruitlessly), unifyWanted traverses the common structure, and
bales out when it finds a difference by creating a new Wanted constraint.
But where it succeeds in finding common structure, it just builds a coercion
to reflect it.
-}

unifyWanted :: CtLoc -> Role
            -> TcType -> TcType -> TcS Coercion
-- Return coercion witnessing the equality of the two types,
-- emitting new work equalities where necessary to achieve that
-- Very good short-cut when the two types are equal, or nearly so
-- See Note [unifyWanted and unifyDerived]
-- The returned coercion's role matches the input parameter
unifyWanted :: CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted CtLoc
loc Role
Phantom Xi
ty1 Xi
ty2
  = do { TcCoercion
kind_co <- CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted CtLoc
loc Role
Nominal (HasDebugCallStack => Xi -> Xi
Xi -> Xi
tcTypeKind Xi
ty1) (HasDebugCallStack => Xi -> Xi
Xi -> Xi
tcTypeKind Xi
ty2)
       ; TcCoercion -> TcS TcCoercion
forall (m :: * -> *) a. Monad m => a -> m a
return (TcCoercion -> Xi -> Xi -> TcCoercion
mkPhantomCo TcCoercion
kind_co Xi
ty1 Xi
ty2) }

unifyWanted CtLoc
loc Role
role Xi
orig_ty1 Xi
orig_ty2
  = Xi -> Xi -> TcS TcCoercion
go Xi
orig_ty1 Xi
orig_ty2
  where
    go :: Xi -> Xi -> TcS TcCoercion
go Xi
ty1 Xi
ty2 | Just Xi
ty1' <- Xi -> Maybe Xi
tcView Xi
ty1 = Xi -> Xi -> TcS TcCoercion
go Xi
ty1' Xi
ty2
    go Xi
ty1 Xi
ty2 | Just Xi
ty2' <- Xi -> Maybe Xi
tcView Xi
ty2 = Xi -> Xi -> TcS TcCoercion
go Xi
ty1 Xi
ty2'

    go (FunTy AnonArgFlag
_ Xi
w1 Xi
s1 Xi
t1) (FunTy AnonArgFlag
_ Xi
w2 Xi
s2 Xi
t2)
      = do { TcCoercion
co_s <- CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted CtLoc
loc Role
role Xi
s1 Xi
s2
           ; TcCoercion
co_t <- CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted CtLoc
loc Role
role Xi
t1 Xi
t2
           ; TcCoercion
co_w <- CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted CtLoc
loc Role
Nominal Xi
w1 Xi
w2
           ; TcCoercion -> TcS TcCoercion
forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> TcCoercion -> TcCoercion -> TcCoercion -> TcCoercion
mkFunCo Role
role TcCoercion
co_w TcCoercion
co_s TcCoercion
co_t) }
    go (TyConApp TyCon
tc1 [Xi]
tys1) (TyConApp TyCon
tc2 [Xi]
tys2)
      | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2, [Xi]
tys1 [Xi] -> [Xi] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [Xi]
tys2
      , TyCon -> Role -> Bool
isInjectiveTyCon TyCon
tc1 Role
role -- don't look under newtypes at Rep equality
      = do { [TcCoercion]
cos <- (Role -> Xi -> Xi -> TcS TcCoercion)
-> [Role] -> [Xi] -> [Xi] -> TcS [TcCoercion]
forall (m :: * -> *) a b c d.
Monad m =>
(a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m [d]
zipWith3M (CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
unifyWanted CtLoc
loc)
                              (Role -> TyCon -> [Role]
tyConRolesX Role
role TyCon
tc1) [Xi]
tys1 [Xi]
tys2
           ; TcCoercion -> TcS TcCoercion
forall (m :: * -> *) a. Monad m => a -> m a
return (HasDebugCallStack => Role -> TyCon -> [TcCoercion] -> TcCoercion
Role -> TyCon -> [TcCoercion] -> TcCoercion
mkTyConAppCo Role
role TyCon
tc1 [TcCoercion]
cos) }

    go ty1 :: Xi
ty1@(TyVarTy TyVar
tv) Xi
ty2
      = do { Maybe Xi
mb_ty <- TyVar -> TcS (Maybe Xi)
isFilledMetaTyVar_maybe TyVar
tv
           ; case Maybe Xi
mb_ty of
                Just Xi
ty1' -> Xi -> Xi -> TcS TcCoercion
go Xi
ty1' Xi
ty2
                Maybe Xi
Nothing   -> Xi -> Xi -> TcS TcCoercion
bale_out Xi
ty1 Xi
ty2}
    go Xi
ty1 ty2 :: Xi
ty2@(TyVarTy TyVar
tv)
      = do { Maybe Xi
mb_ty <- TyVar -> TcS (Maybe Xi)
isFilledMetaTyVar_maybe TyVar
tv
           ; case Maybe Xi
mb_ty of
                Just Xi
ty2' -> Xi -> Xi -> TcS TcCoercion
go Xi
ty1 Xi
ty2'
                Maybe Xi
Nothing   -> Xi -> Xi -> TcS TcCoercion
bale_out Xi
ty1 Xi
ty2 }

    go ty1 :: Xi
ty1@(CoercionTy {}) (CoercionTy {})
      = TcCoercion -> TcS TcCoercion
forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Xi -> TcCoercion
mkReflCo Role
role Xi
ty1) -- we just don't care about coercions!

    go Xi
ty1 Xi
ty2 = Xi -> Xi -> TcS TcCoercion
bale_out Xi
ty1 Xi
ty2

    bale_out :: Xi -> Xi -> TcS TcCoercion
bale_out Xi
ty1 Xi
ty2
       | Xi
ty1 HasDebugCallStack => Xi -> Xi -> Bool
Xi -> Xi -> Bool
`tcEqType` Xi
ty2 = TcCoercion -> TcS TcCoercion
forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Xi -> TcCoercion
mkTcReflCo Role
role Xi
ty1)
        -- Check for equality; e.g. a ~ a, or (m a) ~ (m a)
       | Bool
otherwise = CtLoc -> Role -> Xi -> Xi -> TcS TcCoercion
emitNewWantedEq CtLoc
loc Role
role Xi
orig_ty1 Xi
orig_ty2

unifyDeriveds :: CtLoc -> [Role] -> [TcType] -> [TcType] -> TcS ()
-- See Note [unifyWanted and unifyDerived]
unifyDeriveds :: CtLoc -> [Role] -> [Xi] -> [Xi] -> TcS ()
unifyDeriveds CtLoc
loc [Role]
roles [Xi]
tys1 [Xi]
tys2 = (Role -> Xi -> Xi -> TcS ()) -> [Role] -> [Xi] -> [Xi] -> TcS ()
forall (m :: * -> *) a b c d.
Monad m =>
(a -> b -> c -> m d) -> [a] -> [b] -> [c] -> m ()
zipWith3M_ (CtLoc -> Role -> Xi -> Xi -> TcS ()
unify_derived CtLoc
loc) [Role]
roles [Xi]
tys1 [Xi]
tys2

unifyDerived :: CtLoc -> Role -> Pair TcType -> TcS ()
-- See Note [unifyWanted and unifyDerived]
unifyDerived :: CtLoc -> Role -> Pair Xi -> TcS ()
unifyDerived CtLoc
loc Role
role (Pair Xi
ty1 Xi
ty2) = CtLoc -> Role -> Xi -> Xi -> TcS ()
unify_derived CtLoc
loc Role
role Xi
ty1 Xi
ty2

unify_derived :: CtLoc -> Role -> TcType -> TcType -> TcS ()
-- Create new Derived and put it in the work list
-- Should do nothing if the two types are equal
-- See Note [unifyWanted and unifyDerived]
unify_derived :: CtLoc -> Role -> Xi -> Xi -> TcS ()
unify_derived CtLoc
_   Role
Phantom Xi
_        Xi
_        = () -> TcS ()
forall (m :: * -> *) a. Monad m => a -> m a
return ()
unify_derived CtLoc
loc Role
role    Xi
orig_ty1 Xi
orig_ty2
  = Xi -> Xi -> TcS ()
go Xi
orig_ty1 Xi
orig_ty2
  where
    go :: Xi -> Xi -> TcS ()
go Xi
ty1 Xi
ty2 | Just Xi
ty1' <- Xi -> Maybe Xi
tcView Xi
ty1 = Xi -> Xi -> TcS ()
go Xi
ty1' Xi
ty2
    go Xi
ty1 Xi
ty2 | Just Xi
ty2' <- Xi -> Maybe Xi
tcView Xi
ty2 = Xi -> Xi -> TcS ()
go Xi
ty1 Xi
ty2'

    go (FunTy AnonArgFlag
_ Xi
w1 Xi
s1 Xi
t1) (FunTy AnonArgFlag
_ Xi
w2 Xi
s2 Xi
t2)
      = do { CtLoc -> Role -> Xi -> Xi -> TcS ()
unify_derived CtLoc
loc Role
role Xi
s1 Xi
s2
           ; CtLoc -> Role -> Xi -> Xi -> TcS ()
unify_derived CtLoc
loc Role
role Xi
t1 Xi
t2
           ; CtLoc -> Role -> Xi -> Xi -> TcS ()
unify_derived CtLoc
loc Role
Nominal Xi
w1 Xi
w2 }
    go (TyConApp TyCon
tc1 [Xi]
tys1) (TyConApp TyCon
tc2 [Xi]
tys2)
      | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2, [Xi]
tys1 [Xi] -> [Xi] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [Xi]
tys2
      , TyCon -> Role -> Bool
isInjectiveTyCon TyCon
tc1 Role
role
      = CtLoc -> [Role] -> [Xi] -> [Xi] -> TcS ()
unifyDeriveds CtLoc
loc (Role -> TyCon -> [Role]
tyConRolesX Role
role TyCon
tc1) [Xi]
tys1 [Xi]
tys2
    go ty1 :: Xi
ty1@(TyVarTy TyVar
tv) Xi
ty2
      = do { Maybe Xi
mb_ty <- TyVar -> TcS (Maybe Xi)
isFilledMetaTyVar_maybe TyVar
tv
           ; case Maybe Xi
mb_ty of
                Just Xi
ty1' -> Xi -> Xi -> TcS ()
go Xi
ty1' Xi
ty2
                Maybe Xi
Nothing   -> Xi -> Xi -> TcS ()
bale_out Xi
ty1 Xi
ty2 }
    go Xi
ty1 ty2 :: Xi
ty2@(TyVarTy TyVar
tv)
      = do { Maybe Xi
mb_ty <- TyVar -> TcS (Maybe Xi)
isFilledMetaTyVar_maybe TyVar
tv
           ; case Maybe Xi
mb_ty of
                Just Xi
ty2' -> Xi -> Xi -> TcS ()
go Xi
ty1 Xi
ty2'
                Maybe Xi
Nothing   -> Xi -> Xi -> TcS ()
bale_out Xi
ty1 Xi
ty2 }
    go Xi
ty1 Xi
ty2 = Xi -> Xi -> TcS ()
bale_out Xi
ty1 Xi
ty2

    bale_out :: Xi -> Xi -> TcS ()
bale_out Xi
ty1 Xi
ty2
       | Xi
ty1 HasDebugCallStack => Xi -> Xi -> Bool
Xi -> Xi -> Bool
`tcEqType` Xi
ty2 = () -> TcS ()
forall (m :: * -> *) a. Monad m => a -> m a
return ()
        -- Check for equality; e.g. a ~ a, or (m a) ~ (m a)
       | Bool
otherwise = CtLoc -> Role -> Xi -> Xi -> TcS ()
emitNewDerivedEq CtLoc
loc Role
role Xi
orig_ty1 Xi
orig_ty2